I use this free utility which tells me exactly what's going on with my two video cards:
http://www.evga.com/precision/default.asp
Before I assigned a Custom Profile to CoD one of the GTX 560 Ti cards was doing about 90% of the work (and running a high temp) and the 2nd card was loafing along at 10% and a low temp. With the Custom Profile they're now sharing the workload evenly with moderate temperature rises roughly equal (the upper card runs about 10 C warmer).
This utility also enables you to program the video card fans to automatically increase in speed (for better cooling) as the cards' workload increases.