###Yes - legendary item effects stack across multiple pieces of armor.
As discussed here on the Fallout Wikia page for Legendary Armor Effects (my emphasis):
Legendary effects can be added to items by dropping them on the ground and using the console command "attachmod" while having them selected, followed by the mod ID. You can't stack several mods on a single item, but you can stack over multiple pieces (armor). You can also replace a mod with another one via console.
This means that if you have multiple pieces of armor with the same legendary effect, these will stack together.