uhhh Yes you can, have you never heard of SLi or CrossfireX, the high end 17" Alienware has 2 ATI GPU's working together for 1 screen.
i am not really an engineer or techie, but, i was thinking of crossfire and sli, too and wondered why they can't do it with an integrated gpu and dedicated gpu using the open cl standard.
my initial (nonexpert) thought is that intel drivers can't "talk" work w/ drivers from amd/nvidia.
but, a recent article today has me thinking if this could be possible in the future, either from apple or windows, since neither platforms takes advantage of the integrated graphics to work at the same time with the dedicated graphics. currently, this is only possible in sli or corssfire configs with 2 dedicated gpu's in the windows platform.
article here:
http://www.engadget.com/2011/03/01/amd-compares-upcoming-llano-fusion-apu-with-intel-core-i7-kil/
so, perhaps, if AMD ever puts that out, apple can make Macbook (Super) Pros with SLI/Crossfire-like capabilities. they would have to adopt AMD's platform, of course, which is similar to Intel's Sandy Bridge w/ inetgrated GPU's, except, AMD's uses AMD's own GPUs and if, future vendors such as Apple puts a dedicated AMD GPU, then, wouldn't 2 AMD GPU's (inetgrated + dedicated) be easier for OS x or Windows to leverage both graphics to work at the same time?