Use Deadly Poison to buff main-hand weapon, because most spells are stroke by main-hand weapon. Deadly Poison Effect can be stacked, up to 5 times. So use Deadly Poison can make highest damage.
Use Crippling Poison to buff off-hand weapon, Crippling poison can reduce opponent's moving speed. Due to off-hand weapon speed is kind of slow, even though using damage poison wouldn't get too much bonus damage. Crippling Poison can last for longer time, it can't do damage, but it can stop humanoid mob's from running away or kite mob very efficiently.