She is probably eating it while you are asleep.
But if you think that Lily is taking it all for herself, you can put 2 plates in different positions in their cage. It would be impossible for Lily to guard both plates and impede Finley from getting to want of them.
The only way for you to make completely sure they are both eating it, would be by staying up and observing them or leaving a camera (that has night vision), but I doubt you'd want to do so.
As I said, I believe she is eating it while you asleep, gliders don't eat much at a time, but would rather eat several times at night, and during those feedings it is very possible that she is eating the staple, like she used to when she was alone.
Hope any of this is helpful.