My solution for this is to use the new Spaces feature in OSX 10.5. Basically, i just put parallels in Single Window mode, Drag it accross both display desktops, then move the window into another "Space." Whenever i need XP, i just scoot over to that display and it's available as a big XP desktop spanned accross both windows.
Of course, this does not work with Coherence, and it probably isn't exactly what most readers are looking for, but it works for me.