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).
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).