I'd like to read more about this setup!
Cliff notes:
It's basically an extension of the Dual CPU setup mentioned in http://www.gtoal.com/sbt/
In that article, Graham Toal discusses a hack where you can bootstrap a new emulator core from an existing one by making a custom emulator that drives both cores at the same time.
After each clock tick, you check the states of both processors for divergence and halt with the diff if there is one.
Makes me consider getting back to lower level coding again, even just test code, just to see it in action.
Using it, and a suitable test program, you can bootstrap an emulator core for something simple like a mips, z80 or 6502 in a day or two.
I highly recommend it as a low-level itch scratching programming exercise.