You should never, ever, give them the chance to do that. Better keep a close eye on their mouths. Check daily for a while. Look for swelling around the "seal". Look for gaps, lumps, etc.

More than once I have had to treat (and it's not easy to do or quick to heal) mouth injuries from this. Treat at the first sign of trouble and the chances for recovery are good. Do nothing, and it could eventually lead to tooth loss, infections, mouth pain which causes them to stop eating, and possibly death.

Do everything you can to prevent it from happening in the first place. If it does happen, tapping firmly but gently on their heads should get them to let go. Never attempt to pull them apart.

This should be taken very seriously. There's nothing funny about it.