In any case: The right way to negotiate for an office is not when you're initially hired, but when you've been there a while, have somehow come into possession of a fair amount of bargaining power against them, and are renegotiating your deal.
The same as if I really cared about any other accommodation (like a salary). I wouldn't trust my salary to a handshake deal.
Edit: I'm not at either of those jobs anymore, but I think if I had tried to negotiate for a shared office after the company had moved to an open office plan, I would have been laughed out of the room.