Cannibalism is inevitable when you keep certain species in enclosed spaces.
You'll notice that they'll eat each other's heads off even if they're well-fed. It may be their way of decreasing the competition for prey items in their own territory.
You cannot keep those together. I've come across female on female cannibalism in the wild a couple times. Once I found one that was caught and eaten by another female while she was laying an ooth.
chinese mantises are not communal like most other species of mantises you have to have close supervision if you have 2 out but even with that that is a huge risk i wouldn't take.
Almost no mantises are not cannibalistic. Most mantis are not cannibals at early stages, but I wouldn't even take that chance. Mantises that are commutable include Ghost mantis, Giant African stick mantis, and a few more.
Although many mantids are rather tolerant of each other, there are no truly communal mantids; they are solitary animals. Every species has a risk of cannibalism. Tenoderas are a particularly aggressive and uncommunal species, who will attack anything in sight.