Yeah, I was talking about the actual Highlander mythos, by way of explaing my problem with having the power bound in the sword. As much as each of them has a sword, that specific sword is clearly not a needed thing, and normal people can't get Quickening from beheading an immortal with the sword of one. It's the immortals who are special, not their weapons.
As for what I'd suggest, well, like I said, the fight with another Immortal shouldn't be a casual thing, it should be a big enough story element to be a Milestone...which neatly covers the gains from doing it. No need for complicated and unbalncing mechanics when the system has a built-in way to do it with basically no modification.