What are you trying to do with the macro? If you just want to arm/disarm your shield you could either make a macro to arm/disarm left then right hand OR make two macros to unequip and equip respectively, ie,
arm/disarm (single hotkey)
1. Arm/Disarm (left hand)
2. Delay (1.0) <-- depends on your lag, try tweaking it
2. Arm/Disarm (right hand)
Only issue with the above is if the delay is too short you'll get the 'you must wait to perform another action' jazz and it won't do the right hand. So a safer route could be the two macros, one to equip both and one to unequip and hammer them until they work.
Keep in mind the Delay action is pretty flaky (combined with targeting anyway).
I really wish we could just have a single 'equip' action that drops anything currently in hand and equips whatever.
Someone please tell me there's a better way