I assume it should be fine since there are 2 3.0 slots. You can do it in the following: 2 (x16/0 or x8/x8)
As long as it fits, it'll work.
Even if they were 2.0 slots, it would work fine (backwards compatible) Hell i'm using a 2.0 card in a 3.0 slot.
Im waiting for the 7950 to drop before they release its replacement and i may consider getting 2 myself in this very same board. Personally they always recommend just getting the 400 dollar card which would be the equivalent of 2 cards since:
only 1 point of failure instead of 2.
However i always like having the option of adding a second card which is why i bought the -V board and it supports full 16/0 on 2.