6 x 4K - One machine, multiple GPUs

Hi folks,
hope this is the right place for my question. We are planning a Windows/Unity based system for multi projection at ultra high resolution. The traditional way is to run a GP cluster, but looking at the development in GPU performance we wondered if the time has come for a monolithic machine.
What we need is 6 screens in 4K plus 2 additional UI displays.
Is it possible to use 2 or 3 independent high end GPUs for one application? SLI doesn't seem to be the way here: http://www.nvidia.in/object/sli-technology-multimonitor-in.html
Thx, Bastian