I agree with both Jean-Michel and Margaux.  Best to attack this through governance, and the possiblities are endless.  You could even have a "Backup" role accountable for "Performing the accountabilities of roles x, y, and z on a short-term basis in the event of holiday or sick leave of the primary role-filler for that role."

As to your question of the role-filler putting that work back on the other role who raised the tension, it sounds like a valid proposal (and one that would not be easy to object to!).