We set up family sharing between us to access each other’s games, but did that I think entirely on a computer via that steam client. No pins or anything were necessary iirc, just a slightly convoluted sequence of logging in and out of steam on the same computer and clicking the needed ‘family sharing’ buttons.

Just a note that family sharing is per device, so you’d both have to login to the Steam Deck and enable family sharing. You can’t just login to some external computer and have sharing work on the deck