I have never bought more than one gpu, and wont anytime soon - its excessive and not neccessary. Whilst an nvidia gx2 or ati x2 card (dual core single card) would not be out of the question, as neither card require more than one slot, or any specific brand hardware.
You cant say that anyone buying on of these cards will have an sli mobo if they buy nvidia, or a crossfire capable board for ati - most people dont buy dual cards.
Fine if you are reviewing cards in sli vs crossfire, have x48 for ati and 790i for nvidia - thats a justifiable difference in subsystem......but if your doing single card benches keep the subsystem the same.
If you need 'comparable' single card benchmark scores for use in an sli/crossfire review - run the tests again on the seperate subsystems.
Most of your current reader base, if I had to go out on a limb and guess, dont own an nvidia chipset mobo, they will have an intel - and most will have an nvidia card. So testing these new single cards on an intel mobo is actually more relevant to your users anyway, and noone will cry bias or foul play towards one manufacturer or the other.