Nope, you can't run multiple contextual ads from different providers at once in AdSense's TOS (could be the same for YPN too).
On the same page for sure is not allowed - but for different pages in the same site? It can be done to rotate the scripts for YPN and Adsense and in the same time to be shown only one of them.
Actually, that's not true. You can use both on the same site at the same time, you just can't have them showing up on the same page at the same time.