If you want to setup a dual boot that can be done easy enough. Leave one drive unplugged while installing one version on the other then reverse with that one unplugged so each is bootable on it's own.
Once both versions XP, 7 or Vista whichever are on you edit the 7/Vista boot loader to add XP in there. First you have to unhide XP's boot files and copy those over to the C:\ directory on the other then edit the boot.ini(copy) to make a small change like rdisk(0)partition(1) to rdisk(1)partion(1) since XP would be seen on the second drive.
To see the new XP entry added into the 7 or Vista boot loader you can manually use the BCD edit tool included in the newer versions or grab a free tool for that at http://neosmart.net/dl.php?id=1
With 7 on the second partition here with XP on the first I had to go into the Disk Management tool to see XP(boot) given a drive letter so you could access that partition from 7.