after kephess does his knockback, he will start casting an ability. breath of the masters i believe it's called. this causes the tank he casted it on to start dropping purple circles. 4 in story, 5 in hard. if the tank that has this debuff takes any damage, he's dead. KO.
therefore, tank #2 must taunt during this casting bar, so that kephess switches targets immediately after he places breath on tank #1.
and as someone mentioned, tank #1 with the purple circles shouldnt do anything other than place circles, as kephess might do his random attack. and that would result in dead tank #1. from your description of your problem, i think this is the issue. try doing it with tank #1 doing nothing else while he places circles.
Yeah i think this is the problem, we have been taunting after the knock back, and not waiting for the cast.
And to the other post, we havent been putting our backs to the walls, we face an open area so we get thrown back far enough to give the other tank enough time to taunt. but i guess that shouldnt be a big deal since he stops melee to cast.