OK, that helped, sort of. The only way I can get my original performance when backing up DVDs(~1/2hr to back up) is to use the slave for both source and destination. Either method of using one drive for source and the other for destination gives me a 2+hr total backup time.

This seems really odd, as far as I can tell there is no concurrent access to the drives, as expected.

I'll try my options in order from lowest price, first swapping in a new cable that can connect the slave to the middle connector.