The P35 chipset supports crossfire, but only at x16 and x4 (or x8 and x8 for asus boards with crosslinx). If you want a crossfire chipset to run full x16 and x16, then currently the only way to go is w/ AMD 580X like JustJC said. However, I've seen several different articles to suggest that both AMD and Intel are releasing their respective new top-of-the line chipsets next month (AMD 790X and Intel X38), and both of these chipsets will support crossfire at x16 and x16.
Plus, if you really have your heart set on the P35 chipset, Asus should be releasing several new Republic of Gamers Boards based on that chipset any day now.
One more thing, with the P35, X38, and 790X chipsets, you'll be able to take advantage of the extra bandwidth of PCIe 2.0 when compatible GPUs come out. This is the only disadvantage, in my mind, of the 580X chipset.