"Is there a way to see the relationship between two people in another user's tree?"
No -- the widget is set up to be between ~you~ and someone in your tree (with that small-type option to change ID of one of the parties via the 'not you?' option). There is no "you" that you can designate in someone else's tree.
This could be something they'd consider in the future, however. But first I'd like them to eliminate considering spouses of distant cousins as equivalent to blood kin.