After he does Second Head (the tank swap mechanic) wait a couple seconds and precast sprint. That'll always be right before nerve gas. If you get a lucky proc you can get through a nerve gas without losing any cast time.
If raging is up for adds then quelling should be as well. I've had a few aggro issues there as well, though, so just watch your bars and if you absolutely need to then throw a couple of moves at the other add.