Eyefinity requires the connections to be to one card. If you want to mess around with it, even with just two screens, it will save you some trouble of reconnecting cables.
Although... I just had an interesting thought. When you have two monitors connected to a single card, the card's idle clock speeds are increased over that of when just a single monitor is installed, and thus power consumption at idle should go up. I wonder how this is different if you have one monitor connected to each card. Does two cards at their lowest idle clock states use less power than a single card at dual monitor idle clock states? Does Crossfire bring up the idle clock states when two monitors are connected regardless of how they are connected? Some more questions like these are things most reviews don't cover.