You can grow multiple plants in one hydro system, just remember to check on the water level in the tank more often since they will eat faster, the bad thing about 1 tank with multiple plants is in my experience- when something fucks up then everything fucks up- for example rot root (happened to me, air pump broke down in a tank with 6 plants...3 week flower...worse day in my career as grower). If the older plant is bigger, then i would HST or TOP it to stun it a little, so the other one can catch up :) just control it and it will be fine