The first one is to store the pipes, the second one for nozzles, I decided to only get the one for the nozzles, (it's available seperately, and there's also a version with some nozzles included). I tought the one for the pipes was supposed to go on top of the hosestorage, and I would have to get the pipes out everytime I would need the hose.
But the things are supposed to go at the bottom of the hosecompartment; so the hose would fit on top of the pipes (I wouldn't know how well this fits, since I don't have the part, and I think it's already enough of a challenge to get the hose in without it being there.) I believe the lower part is also included with the boom-arm, so if you intend to get one of those later on, I wouldn't bother getting one beforehand.