My two get like that too and I always worry that Monster will hurt Merce. However I've had them for a few years now and no harm has ever been done, Merce is usually able to get away. I always worry that Monster will drown her, in fact one time I even picked monster up thinking he would release Merce but instead he left her dangling.

It does look like they will pop but don't worry she won't.

When separating the two I did it to ensure I wouldn't end up with tadpoles and I used a tank divider instead of putting them in two separate tanks.