Hi phdellgren,
As mentioned above they're both correct. However it varies between the system you use.
AMD uses Crossfire, the ability to use 2-4 GPU's (Graphics Cards) at once in tandem. This means a boost to performance. Although time and time again Crossfire has failed gamers as it is still unstable.
nVidia uses SLi, again, connecting 2-4 GPU's in tandem. However more stable than AMD. As well as this you can set specific graphics cards for tasks.
For example I have a GTX 780Ti and a GTX 570, while I could connect them in SLi, it would be unstable as they're different generations of cards.
Back to the point, I can have the GTX570 look after PhysX, a dedicated card for that would free up the 780 Ti increasing performance.
Again it comes down to your choice in motherboard.
I hope this helps,
Liberatas