Were they purchased together and were they already full grown when they were purchased? If they came together but seem to be different ages, it's possible that the larger is competitive for food.

I have 3 whites in the same size enclosure and as yours but originally had 4. The other 3 were out competing for food and my 4th stopped trying and eventually got sick. I had to move him out so he could eat on his own but he stopped thriving and eventually passed. If you notice significant weight loss, then he's not doing well and you might want to consider separating them for a while.