This has been asked for before, but there isn't a good way to do it right now. Given the price of SD cards, I'd suggest getting additional cards for each camera.
You can have the DISKBOOT.BIN from one version and the PS.FIR (or FI2) from a different version on the same card. If you lock the card, it will autoboot using the diskboot.bin. If you unlock it, you can load the other version with with firm update. If you had both dryos and vxworks cameras, I guess you could have both the PS.FIR and PS.FI2 on them, and firm update might work for each. Keep in mind they will share they same CHDK settings, which is probably not desirable.