Thanks for chiming in. Interesting that your laptop has more than one optical drive, heh.
I forgot to mention - I couldn't find the Nero driver, but did come across a sort of fake ASPI that translates ASPI commands to SPTI, called FrogASPI (because the guy's French, natch). Since ultimately I don't think this is a 'real' ASPI layer, I'm not surprised that it didn't change anything.
Ended up 'fixing' the problem on one of my computers by connecting one of the drives to my motherboard's additional on-board Marvell SATA controller. The other computer remains limited to one drive. I think I managed to find the Nero driver just now, so I'm gonna try that in lieu of FrogASPI, but I'm not hopeful. This isn't that critical, but I just don't like it when things don't work properly!