I would start with one and see if you end up needing another. We have one and it's worked so far. We borrowed another from friends the two times we went on an OOT trip and needed beds for both babies. (We have the Graco Margo PnP from albeebaby.com and love it; it's a great deal and I love that the diaper changer goes up to 25 lbs instead of 15 lbs like most models!)
I'd skip the pnp altogether. We bought the original co-sleeper (full size) and LOVE it. It has a higher weight limit in the bassinet feature than the PNP. Our girls still sleep in theirs together at night, although it's getting to be time to split them up. I definitely wouldn't have need 2 either, it's working out for us to just have one.
We have two. The girls nap and sleep in their cribs from the day we came home from the hospital. The pnp's are at my parent's house. They live 2 hours away and that is what they sleep in when we are visiting them.
Re: PNP question