The C128 was a crazy Frankenstein's monster of a machine,though, with two CPUs (and the disk drive adds another), and all kinds of other expensive additions to support the CP/M mode. It even boots the Z80 first and checks for certain conditions before switching to the 8502 (and in CP/M mode some BIOS calls switches between the Z80 and 8502, I believe to save ROM space by reusing IO code etc. across them).

It's a fascinating but totally bizarre machine trying to satisfy several contradicting goals at once (be a business computer; be a better c64; be 100 percent c64 compatible) and in doing so doing none of them well enough other than perhaps the c64 compatibility (which wasn't perfect, but was very close), but at a too high cost.