If the Pata is PATA (parallel ATA) that's IDE. You have 2 IDE devices per channel (connection on the motherboard, and the cable has 2 connectors on it). If they are set up on the same connector, 1 has to be set as slave, the other as master, via jumpers on the drive.
If that's done correctly, then I would pull one, see if one works by itself, then reverse the process. If it works correctly with single drives, and not with dual drives (with jumpers set correctly) then you have at least isolated the problem, could be something to work with the vendor on.