Zach,
After reading this convoluted thread, I still am not sure if you really want to merge two broken lenses.
I have no good ones to offer, which is a pity, because then we wouldn't need to use USPS.
What I would do, is cut with a fine tooth blade - rotary on dremel or manual. But cut about 3/32" away from desired line for both parts. Then use a sanding machine and carefully sand both parts to the marked lines. Do it slowly, but even so, you may get some melting. Let it harden and cut away excess with a knife.
Once both parts are cut and trimmed, next step is to glue. But what glue? Ideally something clear. Depends on type of plastic the lenses are made from. Maybe a hobby shop has a suitable plastic glue?
Weldoon has many adhesives. 3M have also have many plastic adhesives. Cyanoacrylates (crazy glue) might work. Maybe NAPA has a specific taillight glue?
Have fun!