Armorstand + LeftHanded:1b doesn't work

running the command
/summon ArmorStand ~ ~ ~

{ShowArms:1,LeftHanded:1b}

does not summon a left handed mob/entity, even though it can dual-wield.

I am testing for two armor stands with an item in the main hand:
/testfor @e[type=ArmorStand,c=2,r=5] {HandItems:[{id:"minecraft:stick",Count:1b,Damage:0s,tag:{display:

{Name:"Key",Lore:["to secret lair..."]}

}},{}]}

-why I have to do it this way (not too important):
this command works for both the armorstands, but they are both righthanded and I can't put the item in the other hand, because a player can only put the item in the main hand, so I would need an external command block, which isn't even worst. As you can see, does the command check for an item in the main hand, but if I want a lefthanded and a righthanded one I would need to place another commandblock on another location (the [c=-1] won't work because there are more armorstands in the server than just these two).