Quick question about crossfire

Hi so i have a 4870 right now and im looking to get a second one (yeah i know 500 series are out but w/e) i heard if u crossfire 2 cards u can only get 1 display is this true? im pretty sure it is but anyway im gettign a second monitor and i was wondering if its possible to switch off from crossfire to normal so i can game on a single one and then just watch edit videos while watchign vids on youtube on the second just like that? or would i be better off not crossfiring @ all. and if i dont corssfire what would be my fps boost