Separate them ASAP. They are missing something. Have you feed them anything yet? They look hungry to me. Lack of air circulation could do it also. There should be a light fan blowing over them. Read up. Those plants are to young to be having problems. So I'd say it's a basic problem. Since seedlings don't need to be feed the first 2.5 weeks or so. Those plants don't look to be much older than that.