>>530865565
>The one you don't pick dies
Considering those two have been a pair for centuries and suffered through so much together, I imagine one of them dying would cause the other to become inconsolable, and go nuclear - no model w influence needed, they would become beings of pure hatred.
Just have the off-character bed-ridden, enfeebled, or on life support to give the player character something to fight for - say there's only a fix for one of them, and they don't have the resources to fix both, as justification.