The concentrators can be shut off only from outside the chamber. You can always remove mask and let all O2 inside the chamber. No risk of high levels, even 2 oxygen concentrators are safe. One other thing you could do is to plug the oxygen concentrators in an outlet with remote control and have the remote inside the chamber to power ON/OFF as you wish. Outlets with remote are available in any hardware shop. We can also supply this auxiliary for you.