Depends a little on crossfire optimizations and the game or bench, but I have seen it before. Some games will use both to 100%, others will use the primary a bit more. Remember that the primary also has the task of compositing data from both cards into the single output frame to your monitor, so there's a little bit of overhead there as well.
Side note: Consider upgrading to DDR3 1866 RAM for that system, or see if your 1333 can be OC'd to 1600. Just a suggestion.