F_22 Raptor :
Is it possible to use the full computing power of two GPUs on one monitor?
A lot depends on your system's architecture, but the simple answer is yes.
You can use GPU-z to measure the load placed on each GPU while gaming.
I run two GPUs separately which reach the high 90%s GPU load while gaming. The dual-GPU cards are similar, though a little less powerful, than two individual cards with the same GPUs, but operate using the same SLI or crossfire technology. Most newer games (2009-ish and after) take advantage of SLI technology.