which os are you talking about?
win2k and winxp?
winxp and linux?
some people install two os's to play around with another os. i.e they use windows as there main, and have linux on when they want to learn and play around with the os.
Another reason when win2k some older games didnt work on it. so people would have win98 and win2k on there. they would boot into win98 to play games. and then do everything else in win2k because it was more stable