Crossfiring between 2 PCI Express 2.0 x16 (one running @ x16, the othe

The reason they make the slot a full size one is so you can put a full size card in there and in this case you want to put a video card which you can do and it will work. The thing is you may lose some performance because you won't have the full 16 or 8 lanes to use , you will have 4 lanes and that cuts down on the bandwidth. However the fact that you are in crossfire should help and the loss might not be as much as if you were trying to run just one card in a 4 lane slot.