The off-tank must be prepared to taunt Gara'jal as soon as the main tank is Banished to the Spirit World. This will happen immediately after Voodoo Dolls expires from the main tank, and it should be easy to track.
While Vodoo Dolls is active on a tank, Gara'jal will only attack that tank, and it is not possible for the other tank to take over. Taunting Gara'jal during this time still affects the threat table normally, but the boss will simply not act on this. As such, the off-tank can make sure to be second (or first) on threat by the time the main tank is Banished, at the end of Voodoo Dolls.