Two single-track piers spliced together? Cut one end off each, glue the cut sides together, and carve any lines needed to disguise the seam.
A similar option, but cutting lengthwise, would be two abutments back-to-back.
Or, you could cast a plaster block, and carve the stones into it. If you already have the plaster, a simple box would do for the mold, as you can carve the abutment to whatever shape you wish, and the cost would be minimal. Don't like the results? Throw it out, all you wasted was a few cents worth of plaster and some time.