When the CM-64 is in overflow mode using Cloudschatze's sysex that modifies the field that I discovered, it erroneously sends 8x-form "Note Off" messages as "Note On" to the second module if the velocity is not zero. The solution is to make sure that 8x-form "Note Off" messages always have zero velocity.
Hmm. Is this an example of the bug fixed in the 2.07 MT-32 ROM?
"When an MT-32 receives "8x kk vv" (Note Off) in overflow assign mode, the MT-32 wrongly transmits "9x kk vv" (Note On) instead of "8x kk vv" (Note Off)."
By the way, try using a 02 in the "Overflow SysEx" string. In what type of situation might the resulting behavior be useful.