Mantis nymphs will eat whatever they can catch, so they will be fine with consuming flies, ants, aphids, or whatever wanders across their path, which can also include their siblings. You don't need to move them, but if you wanted to, they can be scooted onto nearby vegetation. Mantids wander around and don't have a "home territory," so even if you move them, they might wander back onto the deck from time to time, which is fine. If the deck is near outdoor lighting that's on at night, the insects drawn to that light can also serve as a food source, which might bring the mantids back to the deck periodically.

Miri