ATI cards, in my experience, play worse with other ATI cards....i had driver conflicts in XP, with the AIW drivers thinking the wrong card was installed if it wasn't set to primary.
AIW cards have to be set as primary, as someone else said, for most of their features to work. This being said, if you're trying to get the features of the AIW card you can't use an older PCI card as your secondary...most older cards, especially from ATI, require being set to primary to work at all.
I *think* if you use a newer PCI card, you can set the AIW as primary, and not have a problem, because newer cards don't need to be set as primary to work. From talking to people, it also seems Nvidia is much better about this than ATI.
I'm currently using an ancient ATI 8 mb PCI card (Rage Pro, i think) with a Geforce 2 mx on dual 21 inchers (Viewsonic and HP), and it's working great. Couldn't make it work at all with an AIW instead of the G2mx, without losing all of the AIW functionality.
How about updating the poll to include an option for people not using double-head cards? A lot of people seem to be doing this with two cards...
Dave.