I'm curious, why not use XP x64 for everything that requires XP? I do...
They need to be installed on different partitions, there's no other way of doing it. You don't need anything powerful to create this set-up, nor do you need a sophisticated boot manager, boot.ini will more than suffice. I don't play around with such software, I tend to just use the windows partition manager and the Linux one when I need something more powerful.
But such a set-up is potential dangerous to personal data on your computer, remember to back-up.