Thank you for detailed description. In case of both Z80s - both working, switching between 8502 and internal or external works, only calling Z80 from Z80 not, trying to get them work both at once was attempt with little hope.
Commodore 64 was great, Commodore 128 is bigger, better, faster and more powerful... Commodore 65 was almost here, now is back in Mega 65 and is more than ever was...
I just read the description of the C64 Z80 cartridge by Ruud Baltissen (I gave you the link in another thread), and my assumption that the external Z80 runs when PhiHi is active is wrong (I think). Both Z80 seem to run when PhiLo is active. So maybe it can work. Sadly I don't have a Z80 cartridge so I can't check it with a logic analyzer.