The crank case is connected together, so really you only need one. Most run two though. Typically you can put a PCV on one side, and a breather on the other, the breather becomes an inlet at that point. In a boat though, usually not done.
Some auto drag racers put two PVC's and no breathers on both sides to help the rings seal better (a poor man's vaccum pump). Of course you then have oil vapor contaminating the mixture, but some feel the trade off is worth it.