You don't have to open both doors when he is casting; you can open the first whenever you want then just open the second when he is around 50%. Makes it a lot easier and you can continue to burn him down while he channels. If signing works for you, that is good, but that is kind of way over complicated to explain to people new to the fight. Everyone just take a color and a turn. That way each person only has to remember two doors - and only one person has to worry about opening letting the others continue dps. Much simpler imo.