Many people have rabbits requiring different diets housed to together. Separate during feeding to make sure each is getting the appropriate amount of pellets/veggies, and then put back together. A bunny really only needs a little bit of pellets, it is more of a vitamin supplement for a pet bunny than the staple of their diet. My bunnies eat their pellets in about 15 minutes, then eat hay the rest of the day.
Hay should make up the majority of the diet. I put huge handfuls of hay in my bunnies litter boxes and have a large hay rack so that if I am gone all day, they will not run out of hay. I also buy hay by the bale because buying in the little petstore bags would bankrupt me in no time! Some feed stores will also sell parts of a bale or let you take lose hay that has fallen from the bales, if a bale is too much for you to store.
I have a very chubby mini rex, and she and her partner only share 1/4 cup of pellets a day. So only 1/8th cup a day for each bun. They eat a lot of hay, probably twice their body size in hay a day. But, they are happy and at a much better weight now.
Also, permanently separating bonded rabbits is very detrimental to their mental health. They become very, very bonded to their cage mates, and to be separated can cause major depression and anxiety. Best to just separate for pellet feeding or feed in different dishes.
--Dawn