FWIW, I just bought a P55A-UD3P mobo today, and it does have the two stacked SATA connectors on the board, and included in the box were two right-angle SATA cables and two straight SATA cables, both blue.
I'm with zagood, in that I don't see how you could use two right-angle cables.
So far, I'm just using the other plain old-fashioned six SATA connectors with straight-connector cables, since I'm running SATA 2 HDDs.
How does one know which supplied cables are to be used with SATA 3.0? Yes, I did RTFM. It only says the box contents include "Four SATA cables" and nowhere else indicates which should be used for the two stacked SATA 3.0 ports.
Criminy, I just re-read the manual, and sure enough, clear as a bell it says, for the "SATA 6Gb/s Connectors, Controlled by Marvel 9128" (the two stacked ports):
"Please connect the L-shaped end of the SATA cable to your SATA hard drive." Doh.