I just came across this old comp.sys.cbm post from Marko Mäkelä:

QuoteYou don't even have to modify the C128 to get both processors running at the same time: just enable the C64 mode and CP/M mode simultaneously, and both processors will be activated.  The only problem is that the addresses and the data from both processors will be ANDed together.  But with some very careful coding, you could perhaps create a machine language program that makes sense to both processors at the same time.

Unbelievable!  It sounds doable considering the number of opcodes the Z80 has... but it seems you'd waste so much time with 'silly' opcodes to get them to work together that you'd be better off with 1 cpu.  But with a small hardware hack, to switch RAM banks in each clock phase (e.g., 8502 bank 0 / Z80 bank 1), you could really have something useful.

Without the hardware mod, it would be a awesome protection/encryption method.  Can you imagine authorities/hackers trying to crack that?!