This might sound antithetical to the privacy-by-default standard, but my wife and I currently share our photo rolls with each other (using Google Photos)–everything we take is automatically shared with the other.
Is there anything in Proton Drive/Photos that would allow this? Even if it’s only available within a family plan or something. I know that with Drive, we could share a directory and everything in that would be available to the other, but I don’t see this as an option in the photos.
Alternatively, I suppose we could both sign in to Proton Drive using a third account just for that purpose, but that’s less ideal.
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).