If they don't stand up to a fan I would use vibrations. When i was growing in the dryer the plants that received vibrations from fan on the the shelf grew strong thick stocks with nodes closer togather. Even music with alot of bass should shake up the cells in the stock enough for it to reenforce itself. I would leave them togather though. It's a great learning experience and isn't much differet from growing with multiple tops since the strongest one is always greedy. Don't feel bad that they stretch. It seems likwe it happens to most growers around "here" Myself includeds. I just transplant a little deeper each time, and be gentle.