As long as you never have more than one male, you shouldn't have to worry about dominance. Four leo's should be just fine in a 40 breeder. I would recommend though that you provide more hides (unless you have large hides like the caves). You should have at least three hides in your 20 L now. Two on the hot side and one on the cool side at a minimum. Ideally, the two hides on the hot side should be exactly the same. My two females preffer different types of hides, so I happen to have two different ones on the hot side. They don't like it when I take one away for cleaning.

Main reason for the hides is that if one gets stressed in the single warm hide, she'll go to the cool hide just to be more comfortable. Also make sure you see them both eating. One of mine isn't very good at hunting so if I drop a bunch of crickets in the viv, the good hunter may get them all before the other has a chance to get one.

And as olstyn said, make sure you have a plan for when one or more gets sick. They will need to be separated.