At the end of the day it doesn’t matter so much if they’re in 2x 2 bays or 1x 4 bay that’s backing itself up. It might give a little extra redundancy and safety to have them on separate NAS but the backup software is what’s going to be doing the heavy lifting here and it shouldn’t really matter whether it’s talking to two different disks/arrays on the same machine/NAS (as long as the NAS allows you to split the 4 drives into 2 different arrays which from my experience they do)