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I guess I am a little odd but I would ask the question this way... How would I expect to be treated in the reverse situation? What would be appropriate?
Then balance that with the bottom line of your needs. What is this situation costing you? How specifically is it infringing on you using the shared space? Is it actually costing you more money?
The express both those chains of thought to the person. If they don't get it you have to consider the more confrontational advice being given.
My guess is they are two caught up in what ever they are struggling with to realize the inconvenience. Perhaps if you point it out gently you can come to some agreement.