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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).
Syncthing.
Just sync your DCIM folders.
You can enable "Ignore Delete" on each phone if you want to ensure photos aren't deleted on the source phone if the destination deletes it (or not).
I sync stuff to a PC at home, so phones don't have to be on at the same time - they'll always sync to the home PC.