Taken from my post to a similar suggestion.
Something like this could be implemented. The greyed out items are the ones from the battle load out and so you can drop any item to change it for the siege load out. That way there is no need to duplicate the items, the sword here is glowing because it replaced the one from the battle load out. Any changes you make to battle loadout such as armor could be visible in siege load out unless you manually change it.