OK. Now i understand what you meant/did when you added a new hard drive.
But putting XP on one HD and Vista on the other should work should as well as having one drive containing two partitions and XP and Vista in its own partition. So that should be ok.
Finally, one secret to (an easier) success is installing the oldest OS first followed by the newest. That was the root of my question. But seems you did that as well.
Sorry can't offer more immedidate help but wanted to give u a quick reply as i saw u responded quickly yourself... I'll look around and if i can find anything might help... will post it. In the meantime, would be curious should you find the solution yourself..pls post it.