Tank 1 has the boss, tank 2 has the elemental.
Tank 1 dies after ~16 stacks.
Tank 2 suddenly has both boss and elemental on him.
Tank 1 gets ressed asap.
Tank 1 instantly taunts the elemental.
By the time tank 2 is going to die (also after 16 stacks), tank 1 will have taunt ready again.
Tank 2 dies.
Tank 1 taunts boss.
Tank 2 gets ressed.
Tank 2 takes the elemental again.
Both tanks have a mob each.

After both tanks have suicided, and both have been ressed. You simply do a swap at some point, to avoid a third death. Cause boss wil die before any of the tanks goes too high.