when you enter a crossfire configuration your system uses the card that is located in the main pcie slot on your mobo as the master gpu, meaning that all 3d applications start through this card and when the card reaches a certain percentage of use the second card kicks in or however many cards you have, crossfire uses what we call AFR or alternate frame rendering to create an image with both cards or however many, this means that while one card does the upper half of the screen the counterpart does the bottom, or odds and evens, each gpu decides according to its firmware, that being said whenever you have crossfire enabled your system distributes an even load of work across both cards