To be honest, I don’t know if Proton Drive will do what you want or not. It probably should, if you both share to the same folder, and make that folder “always available offline”.
But if I understand you correctly, I would use Syncthing to do this. Just set up your phones to share the DCIM folder with each other, and as soon as one of you takes a picture, it will copy over to the other phone (as long as you have a data connection or wifi).
We (well, not me, but someone) airlifted food into Berlin for almost a year. The pier is theater, meant to look like they’re doing something. If they really wanted to help, they would make sure the supplies actually got to the people that need it, and not leave it on the beach waiting for more fish to evolve legs.