It depends on the raid but for the most part, you can most often assist off the off-tank. The main tank usually holds onto the boss which is easily targeted. Adds sometimes pop up that an off-tank needs to pick up and burn down quickly, which is why you assist the off-tank.
This is done with the simple macro
/assist [off-tank's name]
do not use the []
Hit that button and you will automatically target what the off-tank is targeting.