Okay, I think I got it working. I swapped the ends of the IDE cable connecting these two drives. i.e., the master & slave ends of the cable. The burners were not on cable select, so the master/slave relationship is still the same. I've done a successful burn, but of course I won't know for sure until I burn more discs.

I have no idea why this worked. The cables were seated nice and tight to begin with.