These are the plumbing vents.
One is for your grey water waste system and the other is for the black waste. They are installed at the top of each waste piping system to allow atmosphere into their respective waste system. Without them, the sinks and shower would not drain and you could not dump your tanks.
Put a finger over a straw, pull it out of a drink and the beverage stays in the straw until you remove your finger.
There are isometric drawings of both systems for all models in my owners manual.