Is it possible for two rabbits to share one litter box?

no.on mother will kill the other litter when she produces hers if in a small box.

but in a big box they might but chances are they will get into a fight

Well, I know that you mis-understood the question. You can litter box train 2 rabbits to the same box if you train them at the same time and they are young. Older rabbits use going to the bathroom as ways to MARK their territory. That is why even a litter box trained buck MIGHT start spraying all over the house. He is marking his territory.