No, In XP you can run multiple display drivers and do multiple monitors. It one of those things that SHOULD work fine, but may be a little quirky in practice.
Vista has the restrictions because the GPU has to do all the compositing now via the WDDM driver, and evidently having two such drivers running at the same time is probably a royal PITA to deal with. Vista can deal with such a setup, but it would have to run in legacy mode with what is essentially the XP graphics stack. This would make you lose desktop compositing as well as virtualized video memory. In other words, you lose most of the "Vista-ness"