We do that quite a bit in our gym for double back timers, vault timers, tumbling up, etc. Sometimes the top mat may slide, so test it first. Some water spray often sticks the mats together. Readjust before each turn. Throwing a pit pillow on top softens the landing, flipping onto a hard cheese mat may not be the best if you are learning front tucks and landing on your bottom/back. When possible, use old, soft mats.