Two OS's on one Computer?

I have two separate hardrives that both have different versions of XP already on them. I am not computer illiterate but i have never done this before and i am wondering if there could be any problems in doing this. One hardrive is having problems right now but once i fix it i would like to do this. The idea came up once i installed my old hardrive in my new ones place(The new one is not in my computer at all right now, and they have not been in there together at all yet). I would just like to know if it is as simple as making my new hardrive master, and my old one a slave. I would like to keep the OS's on both just in case one crashes, like a backup. Thanks for any advice!