As far as I am aware, no motherboard can do SLI or Crossfire with BOTH cards running at x16. The highest is 16x for slot 1 and then 8x for slot two.
1 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot will operate at up to x8 mode.
There's a lot of boards that can do 2 cards at x16/x16, there aren't a lot of boards that can do 3 or 4 cards at x16/x16/x16/x16, but for 2 cards there's a lot.
Maximus V Extreme
Rampage IV Extreme
There's even one that can do 4 at x16, the ASRock X79 Extreme11.