Dear Miss Manners,
Semi-hypothetical situation: You're writing an e-mail to people you want to interview. You want to ask them about some experiences they had, say, thirty or forty years ago. At the time, they were a couple and you have no information that indicates that they are no longer together. So you write to them both: "Mr. and Mrs. X" (or something like that, depending on circumstances).
You get a response from one of them, using the first person singular throughout. S/he is willing to do the interview. But you really want to interview the other member of that couple, too. How do you ask the person who responded to you about the other person?
(Stop looking at me like that! I told you I was socially inept from the beginning!)