Sure, you will just have to ensure the mantids can't get around/over the divider.
You can make one easily with a large piece of plastic (I've cut dividers from plastic clipboards and empty containers with flat sides (like a empty cat litter pail)), just cut it slowly with a big pair of scissors or tin snips. Then hot glue it into place and you'll be set. I would recommend a plastic that the mantids won't be able to see each other through, even shadows, as this will lead to stress for some mantids.