What happens if two mySQL queries both try to lock the same row at the same time?
This is not a deadlock.

One transaction will simply block -- waiting to acquire the lock. The other transaction will proceed. As soon as the other transaction is done -- either by commit or rollback, the first transaction will proceed.

A deadlock happens when a transaction has acquired a lock on object A, and attempts to acquire a lock on object B at the same time as another transaction has already acquired a lock on object B and is attempting to acquire a lock on object A. Both transactions will then block, waiting on each other. That's the definition of a deadlock: two transactions blocked waiting on a lock that the other has.