Keeping in mind that driver support for 64 bit OS's sucks in the first place, add to that the fact that 64 bit OS's drivers support are generally NOT aimed at gaming, and to that you add that multiboxing is something that most any manufacturer will tell you is just not going to be supported...

Question: Does XP support horizontal span across two monitors on two cards? If not, it's going to suffer the same problem he's describing here.

What it sounds like is Vista fixed the multi-monitor on one card display issue but didn't fix the multi-display issue as a whole.