Females can be kept together, preferably in at least a 10 gallon tank and they should be added at the same time rather than letting one get territory established and then adding a new one she would consider a threat. They should still be watched closely to make sure there are no problems and you should be prepared to have to separate them in case there were.

Two males together, is a big no, EVER. There are the occasional stories of it happening but rarely, and they can seem peaceful for awhile and then suddenly for whatever reason that will change.

Males and females should not be kept together either. Breeders will, from what I understand, put them together only for the mating and then remove the male immediately.