At that size it looks like it is probably a female, males tend to be pretty obvious at a young age.
I would strongly suggest separating them either way though. the smaller one is much thinner suggesting that the larger dominates the food. If it is indeed a female the male will likely be harassing her shortly and she needs to be in much better body condition before mating. Even when you do put them together for mating you should not leave them together, the female needs a break from laying eggs. and if it's just a late developer (possibly due to inadequate access to food because of the larger one) then it needs to be moved anyways to prevent fighting with the other male.