I really don't have enough experience to give a good input on it but my male FBT has been doing amplexus as well, and sometimes will hold my other FBT under the water for a few seconds. Being amphibious though I would have to assume they can hold their breath a little longer than humans, so I myself wouldn't worry unless they were under for more than a minute. How long did your toad hold the other toad? Mine seems to do it for maybe 10 seconds then lets go, then comes back later for round whatever haha. The (I guess) female that was grabbed will then stay under for another split second then come back up and return to normal. Hopefully someone more experienced can chime in though