Equips another weapon to your offhand and Shivs your Target. Have an offhand for all 3 poisons. Be sure to replace the [OFFHAND] with the name of the offhand you wish to equip. Press Alt and tap this macro to put back on your default Off Hand. It should be noted there is a 1 second cooldown on weapon changes.
#show [OFFHAND WEAPON] /equipslot 17 [OFFHAND WEAPON] /cast shiv /equipslot [modifier:alt] 17 [DEFAULT OFFHAND WEAPON]