Yeah, add another bin. You have enough light to grow 1-2 nice little plants(put them in separate pots) about 1.5ft each. Start flowering them at around 7 inches tall(they'll double in height, depending on strain). You'll be able to pull maybe an ounce each if all goes well. Watch your temperatures. 85+ degrees is cause for concern, ideal temp is 70-80.
they will MORE that double in size, they might even triple
make sure you don't run out of space
or just add a third box on top