I wish I could offer advice on whether you should separate them, but my experiences never took me that far (lucky for me). For me, the third feeding station worked.
Do all of the feeding stations have the same food? I had also realized I wasn't putting fruit into the top feeding station of my other cage and that was making my one glider territorial so he could get all the fruit.
Did you try the separate feeding stations over the course of several days/weeks or just a couple times? How far apart were they? Can you get one of those bowls that hangs from the side and put it up top somewhere? Can you put the submissive glider at/on the other feeding station to be sure she knows it's there?
And just in case, what diet are you feeding? There may be food leftover but they could still need more nutrition if it's not a good diet (no judgement here, just trying to think of all options).