According to Anandtech, you board has x16 and x16 if the first two are populated. If three are populated, it is x16, x8 and x8. Keep in mind, a GPU runs only as fast as the slower card, just like RAM. A slower lane can potentially limit it. I haven't seen any tests to prove this, but two of these GPUs must use lots of bandwidth. Each PCIe Gen 2 x16 slot has 500MB/s per lane and you have 16 lanes(x16). So you won't have a problem.
Here is the link.
Asus Rampage II Extremehttp://www.anandtech.com/mb/showdoc.aspx?i=3428&p=2
It is strongly recommended to use the first two PCIe slots. I use my two GPUs in the first two lanes.