:-? Attaching two controllers to the same SCSI chain will be problematic. Most SCSI controllers configure themselves as device number seven, though some use device number zero. If they want the same number, it shouldn't work. Even if they use different numbers, one or the other may not play nicely and give up control of the bus when the other wants to use it.
;-) On the other hand, there should be no trouble using two SCSI controllers in the same 'miggy. Why not just temporarily move one controller to the other Amiga? (Mind you, you'll have to set it not to autoboot, else you'll be trying to boot from both of them. Even if it sorts itself out, it could take a while, and then try to boot from the wrong one.)