With crossfire both (or all) monitors connect to the primary gpu. One of the slots on your mobo will be the primary slot, it's usually (but not always) the one closest to the cpu. Connect all monitors to that card.
It doesn't matter that the cards are from different manufacturers, so long as they are the same series, as yours are. You can't, for example, mix a 6850 with a 69xx or 67xx series. There may be small differences in performance between your cards in primary, idk. But I'd use the XFX card as primary (just personal preference). Can't help with the 3D sorry, don't know if you can mix 3D and 2D monitors. You might have to disable the 3D and have it run 60MHz