One ability at a time is fine! Whatever rules is in place for the character's ability must also be in place for Till aka "can't control well because of emotions" will also be the same for him. Obviously needs ooc permission but I'm sure u figured this!!

If there is any ooc conflict, it can be said that time ran out or maybe he didn't see the full ability so he couldn't copy it BUT OTHERWISE, IF ITS LIMITED TO ONE, THERE DOESNT NEED TO BE A TIME LIMIT. It just needs to be replaced.

If needing an extra limit for fun, I'd recommend maybe he has to take the ability from outside the family, not within.