A shared umbrella requires small adjustments from both people beneath it.

Sharing shelter requires continuous small compromise. One person walks slightly off their line, the other holds the handle higher than is comfortable, both adjust their pace, and neither ends up as dry as they would have been alone. Nobody negotiates this aloud, and it fails the moment one person stops adjusting. Most shared arrangements work the same way, on constant unspoken correction rather than on a fair division agreed at the start.

Look at an arrangement you share with someone and ask who is doing the adjusting. If it is mostly one of you, say so gently and take the next turn.