Sure it will happen. nVidia offers cards that can be put into dual and quad SLI configurations. I believe these are two 7950GX2 that each have two 7900 GPUs.
4 GPUs and 2GB of GDDR3 RAM is quite the configuration.
Putting two GPUs together is far cheaper than trying to double the performance on a single card.
We will see single, dual, and (maybe, just maybe, a quad configuration) of graphics card offerings in the new Mac Pro. The quad requires tons of space, power, and the heat generated is also of concern.