Please keep in mind that, as far as I know, none of these ideas are original. They've been dug out of 60 years of magazines and a few books. So, here is how I built my first (and so far only) crossing. Mine was an 18 inch radius mainline crossing an S-curve industrial track. The techniques work the same, but straight track would be easier to design. Double track is more complicated, because there's more to align. These instructions were originally written for a single-track crossing, then edited for the middle crossing in a double crossover, and now for your single-over-double track crossing. The syntax may look a little confusing, but I hope I got it all straightened out. My crossing has been working for at least 15 years, with no problems.
For a double track crossing as you described, I would recommend building the entire junction on a section of roadbed or a piece of wood of similar thickness, rather than trying to build a crossing and install it like sectional track. If you have good access to the layout, you could build it on-site, but if the first attempt doesn't work it may be easier to start over at the workbench! Start with materials. You will need rail, whatever code you're using, track gauges (3-point and NMRA), PC board ties (mine came from Clover House, other brands should work as well), solder, soldering iron, soldering flux (optional, but it does make the work easier), files, saw (razor or saw blade in motor tool), white or yellow carpenter's glue, straight pins or track spikes (not flextrak nails!), rail cutters (works on the ties as well), and probably thick paper or thin cardstock. Other tools are optional and you can use whatever seems handy. Get PC ties in a strip, not precut to length. Also handy will be a picture of a similar crossing, just to see how the prototype builds one. Micro-Engineering sells the 3-point gauges, and you can use rail from a piece of flextrak if you don't want to buy an entire bundle. For only one crossing, the flextrak actually has an advantage, as you'll see later. For rail, the entire crossing should be the same size, even if the tracks use different sizes. Mine is a code 55 main with a code 40 industrial spur, but the crossing is all code 55.
Lay the main tracks on the roadbed, make sure they are in exactly the right place, and pin them down. Don't lay it permanently, because you'll need to remove it. Take a piece of paper (notebook or printer, something white, thin, and cheap), lay it over the entire track area, and tape it to the tracks so that the part over the crossing can be lifted without moving the other end. Use a pencil to mark the location of all of the rails. I used the side of the pencil and let the yellow paint do the marking! Carefully lift the paper and remove the section of track you just laid. Install the other route, put the paper down and make sure it is still aligned with the original tracks, and repeat the rubbing. You now have a plan for your crossing. Trace over the lines with a pencil or pen if they are hard to see, and you may want to make a copy if you plan on making other similar crossings.
The next step is to decide how to lay the ties. On a shallow-angle crossing, the most common way I've seen is to lay the ties at right angles to the centerline of the crossing, which means that they are at an odd angle to both tracks. If it's closer to 90 degrees, the ties usually followed the main tracks, often with doubled ties under the crossing rails. If you are using flextrak, lay a few standard length PC ties on either side and you can save a little work later. Mine was two plain tracks, at almost a right angle.
PC ties are usually thinner than plastic or wood ones, so you will have to make up the difference. This is where the cardstock comes in handy; just add pieces on top of the roadbed until the ties are the right height. Cut your plan to the right size and glue it on top of the stack (make sure it doesn't make the stack TOO high!). Glue the ties in place, and let everything dry thoroughly.
Take a piece of flextrak and remove any ties that are in the crossing area. If there is track on either side of the crossing, remove the ties in the middle and leave them on the end(s). This way, you know the gauge at the joints will match, as there won't be any joints! You can always trim the track to fit the layout later. 3 to 4 inches of track on either side is plenty, unless you are building it in place and know you will need the rest anyway. You will have to cut a lot of electrical gaps later, but don't worry about them for now. Make sure the ties you left on the flextrak clear the crossing ties, and remove the track. If you removed too many ties, slide the remainder in to fill the gap. When you have everything just right, clean the bottom of the rail and the top of the ties (fine sandpaper works well) and apply some flux to the bottom of the rails if you're using it. Put the track back in the rail joiners and pin it down at the other end of the crossing. Using the 3-point gauges, pin the rails in place through the crossing, making sure you have no kinks or unwanted curves. When satisfied, solder one rail and recheck the gauge. Make any needed adjustments and solder the other. You can remove the pins and test the track by hand. A car pushed through it should roll smoothly, just like any other plain track (which it is, so far!).
The rails should be centered over the lines on the paper. Make sure the crossing lines still line up with their track, and then get your file. I used a knife-edge needle file. A cutoff disk or saw could be used, but I wanted more control. File through the head and halfway through the web of the rail at each place the rails are to cross. If done carefully, the grooves will be directly over your lines. Take another piece of track, remove the ties and check clearances as before, and lay it over the first tracks, aligned with the crossing track. Mark the rails where they cross. One of the tricky parts is that you need to file matching slots in the BOTTOM of these rails, so the two sets will interlock. You can't just lay the second track on the first upside-down and file, because the grooves will point the wrong way! (Unless you're building a 90 degree crossing, and you can BUY those). It took me a couple tries to get mine exactly right.
Once they fit together recheck the gauge of the second track. If it isn't exactly right, you can file the slots in the first track to fit. When everything is right, clean and flux these rails, gauge and pin them, and solder them as you did the first set. Make a final check of all of the gauges and alignments, and then solder the frogs. Prototype frogs are heavy castings, so fill the outside corners well with solder. This crossing should last forever. File or cut the flangeways and test the crossing by hand. Note that by interlocking the rails this way, the frogs can't come apart or get out of alignment, no matter what happens.
When you're satisfied, start adding the guardrails. The outside ones are easy, just file one end of a piece of rail to fit against the running rail, file or bend the other end to collect the flanges, and solder them in place. Use the NMRA gauge to get the flangeways right. Basically just like a turnout. The inside guardrails are trickier. You can cut and file 4 pieces of rail, but then you have to get them to match each other at all four corners. I made mine out of one piece, with the corners filed through and bent. But it is about an 80 degree crossing, so the center piece is almost square and the angles are similar. On a shallow angle crossing, it may be easier to use two pieces with a shallow bend in their middles, and file the ends to fit like building a switch frog. You may want to try more than one way here, as rail isn't that expensive. Solder the guardrails in place, making sure the flangeways are right, and clean out any excess solder from the flangeways. Cars should now roll through on their own, without picking the points on the frogs. If not, either the gauge or the flangeways are wrong. If everything works, you can start gapping the rails.
If you are building the crossing at the worbench, now is a good time to lay it, and solder it to the connecting tracks. It will be easier to cut the gaps without knocking something loose if the ends of the rails are solidly fastened. There are two types of gaps needed here: in the ties and in the rails. All of the ties have to be gapped down the center of each track to prevent shorts. The running and guardrails also have to be gapped in the center of the crossing for the same reason. It is easiest to cut both sets at the same time with a saw blade in a motor tool, but a razor saw will work. For the ties, you can use a knife and remove foil from the middle if you like rather than cutting a thin slot. Either way, when done you should be able to apply power to either track without a short. Like a turnout, the two rails connecting to each frog are still electrically connected outside the crossing. If you want insulated frogs you're on your own! You may want to buy or borrow a book on model railroad wiring to connect the crossing.
The second set of gaps is to isolate the crossing from the rest of the layout, and involves cutting all 8 rails outside the frogs. This makes the crossing its own block electrically and also requires gapping the ties between the tracks from these gaps outwards. Each frog should now be electrically isolated from any other track, and the four connecting tracks should be isolated from each other and insulated down their centers. Try to cut these gaps 3 or 4 ties in from the outside end, so that the connecting tracks are soldered to the crossing assembly. This will keep everything lined up and help prevent the gaps from working closed. If you have to remove these pieces to solder the crossing into the layout later, just clean up any lumpy solder and solder the new rails in place.
Wiring for manual operation is fairly simple. You will need a 4-pole double throw switch (Peco sells one, and I'm sure others do as well). Connect a feeder to each of the four frogs, a little extra solder on the outside won't show, especially if you make the connection right against the outside corner of the frog. Wire each frog to the center connection of one pole on the switch. The two outside connections on that pole will go to the two connecting rails for that frog, so that the entire crossing is connected to the correct track for each direction. Drawing it out on paper makes it a lot easier to understand. If you want automatic switching you're on your own, as I have no idea how to do that. I'd stick with the manual switch, and put it as close to the crossing as possible for easy operation.
I hope these instructions work for you. Mine hasn't given me any trouble, and I've run everything from a unit train of Atlas ore cars to 85 foot passenger cars with mixed body and truck mount couplers through it. The most important thing is to make sure it's right even if you have to do it over. That may be annoying, but not nearly as frustrating as years of derailments! On the bright side, a crossing has more parts but is actually simpler to build than a turnout because there are no moving parts. Once it is right, a part stays where you put it.
Here's an old picture showing my crossing in place.
