In fact the 8520 CIA chips used to control the joystick ports on the Amiga were compatible enough you could put them in a C64 in place of one or both 6526's... (You'd lose the realtime clock, as the 8520 had timers instead, but "nobody" used the realtime clock on the C64).

I tested that at one point when one of the 6526's on my C64 broke. I don't think the reverse would work, as I think AmigaOS used the timers, but never tested.

(My parents were not amused that I opened the machines and swap chips around...)