I was actually hoping to find out what he meant because i don't have a complete grasp on SLi and i thought i was trying to raise a valid point.
But the norm in OcUK Graphics, things got out of hand
Put simply, 2 GFX cards work together, the first one renders a frame (or part of it) then the second card renders the other frame or part.
Essentially like 2 people digging a hole with shovels, they cant shift more than a shovel full each but they can shift theoretically twice as much working together
If you consider the memory as the shovels, they are not combined so you can only ever shift as much a 1 shovel can hold at a time as only one can be in the hole at a time.
So basically 2 GFX cards could run higher resolutions and AA etc because you are doubling the frame speed but that territory could be limited by the memory.