You could but because of the difference in the Boot manager you would have to install Windows 7 last because Windows XP does not use the same boot manager. But the real question is since you already have Windows 7 why would you even need XP? I just do not see the need for a 10 year old OS at all.
I have to run a windows XP VM (xp mode) at work, because some business portals only work with older versions of IE such as IE7, etc. Compatability mode doesn't work with every site, and Windows 7 won't take an IE 7 install that I know of. The XP mode thing is a free download and its relatively easy.
I could be wrong, but if you have to have XP, this is a good fix and its working wonders for me.