i seperate mine as soon as they hatched.. but when i ran out of tubs i put at least 2 geckos in one tub.. then if i see the other gecko is getting bigger to the other one i seperate them..
Sometimes you can keep them together for while longer. As long as they arent fighting and their enclosure is the appropriate size. Even males that are raised together often will not fight until exposed to a female, or when they reach sexual maturity which is around 30 - 35 grams.. they sometimes can get territorial.
you can keep them together as long as 1) there's no competition/one isn't noticeably larger than the other, and 2) they are too young to sex
as soon as you can sex them visually, they are at the age where they shouldnt be kept together.. we have kept some of ours together up until they were about 25 grams