Super awesome that they started doing it at the same time!! LoL
When my little guys start trying to do this and I feel like they still need to be in a playpen; I will put them down for a nap and either stay in the room or leave the door open a crack so I can watch/listen. AS SOON as they make a move to climb out, "NO. It's naptime." Lay them back down. Stern face and voice.
Sometimes you only need to do it once, sometimes it will feel like a thousand times!! But, in my experience, they figure out that there is no gymnastics involved in naptime.