Earlier quoted context omitted.
I had assumed that they had some kind of obtuse M of N scheme with no sufficient number of parties in the same place at once, but that was perhaps too optimistic (?) of me.
Armed government agents show up to your door asking you for half a billion dollars. You tell them "can't do it, I need this other key that's held by my partner in a different place". What do you think is going to happen next? "Understandable, have a good day" and they go home? Or they hold you in jail until you somehow find a way to comply?
if such an event happens in a more or less civilized country, you might eventually get some third party to confirm that you're unable to disclose the key