Something kinda different, but.
Years ago I was at a convention with a friend, I say friend, it was work related, but we got to be close later, I'd actually travel to his place and hunt, he would come to mine (more than your normal days work to get between them).
Anyways, point being neither of us knew much about anyone else there, he turned to me and told me the password on his phone incase I'd need to open it to get in contact with his family for an emergency. At first it seemed weird, but after thinking about it made a lot of sense to me.
Probably something that if it's 2 people out together, be good to be able to use the others communication device if needed, like yours becomes inoperable for some reason.