Na, the P55's are full ATX standard.
I'm using a P55 and was thinking of doing the same thing but as AlienALX has said, it would depend on the PCI-E slot layout. You could in theory run them straight on top of each other but the top card would get very very hot.
If you were wondering about whether the slots could handle it, again some of the boards run 8 x 8 on the PCI-E lanes and others split them differently. However, if it is 8 x 8 then that would provide enough bandwidth for the cards.
CPU: Intel Core i5 2500K @ 4700mhz - 47 x 100MHz
Mobo: MSI P67A GD65
GPU: EVGA GTX 570
RAM: 2 x 4GB Mushkin LV 1.35v 1600MHz DDR3
HDD: 3 x 1TB Samsung Spinpoint F3's
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