Are they on the same cable? If so then this may very well be your problem. I run my DVD player on the ATA66/100 cable that is on my hard drive. While not ideal for DVD, I don't watch movies, I really just wanted to have a quieter, better running CD-Rom drive and thought maybe a DVD might be a longer lasting solution for me. I seem to go through CD-Rom drives, they are the only component that seems to truly hate me, they always fail right after their warranties are up, and some have annoyed me so bad (Creative 52X) that I wanted them to fail early so I could replace it........
..............edit..............
Forgot to ask if DMA was enabled on those two drives. Also have you installed any newer chipset drivers for your motherboard?