I've got an ACA1220 Accelerator, that wouldn't boot with an ACA500Plus, but when reading the manual it suggested pulling it out a couple of mm, which did the trick
What I noticed while I had the ACA1220 disabled was that my current CassicWB CFcard wouldn't boot, error saying "Stack Overflow" but when I got the card working it booted fine ?
Also I cannot select Action Replay Roms with the ACA1220 connected, is this normal ?