Generally speaking, P55 isn't as good as x58 for running more than one card. I think just about all x58 boards have 2 x16 slots while most P55 have one x16 and have a x8 or x4 in another. You may not notice much of a difference between the two though.
Actually, I just read that Asus has a P55 board that has 2 x16 slots (And USB 3 and 6GB Sata). I read somewhere that P55 didn't support this. I guess what I read was wrong.
P55 doesn't support two full bandwidth x16 slots with the on-die PCIe controller. However if you add an external PCIe controller you can add extra lanes. This is how it was done in the past, before PCIe on-die was available.