First, just to be sure, try each of the 80 GB drives alone on the end of the IDE cable, setting both to Master. That's to be sure both drives actually work OK. As you try each, take careful note of what the BIOS says about which parameters it is using for each drive.
Next, check the drives themselves for labels on how the jumpers are set, or go to WD website for these exact drives. A few drives (I don't know whether these particular drives or not) had different jumper settings for Master with no Slave, or Master with Slave. Or maybe yours don't care about that, and there's only one Master setting and one Slave setting.
So, hook up with Master on the end, Slave in the middle, but you already know that and said you did it and it did not work. But hook them up and look in the BIOS screen to see what parameters it is trying to use for each drive. They should match what it said when the drives were successfully tested (above) by themselves. If not, try the BIOS screen that auto-detects drives and see if that fixes it. Failing that, you can always manually set the drive parameters the way that worked before, and see if they will both work that way.
Another route: set both to "CS" (Cable Select) and be sure to connect the end of the cable to the drive you want as Master on the IDE line. (Since you're not booting from them anyway, this may not matter. But the one on the end WILL be the Master when you use CS.)
If that still doesn't work, suspect a bad cable (replace) or worse, a bad mobo connector or controller. Don't like that last thought!