I'm not an adcom, but my significant other and I are both applying this cycle (and recently found out that we share an acceptance at the same school
🙂 ), so I do know a little bit about this type of situation.
I never mentioned my s/o during an interview, and I would not recommend doing so because you don't want the interviewer to think that you applied to the school just because he did. That would be REALLY bad. On that note, I wouldn't recommend applying to a school just because your boyfriend is applying, because it could cause some resentment down the road. My s/o and I made our list of schools without consulting each other at all, but we just happened to have some overlap because we have similar stats and interests.
I remember reading a thread this summer where
LizzyM said that once one person in the couple is accepted at a certain school, it may be appropriate for the other person to send a letter of intent to the school and possibly mention the fact that they would love to go to the same school as their significant other who has already been accepted. In this case, the school may be a little more likely to accept the applicant because they have a high chance of matriculating. However, I think that that only really applies when you are engaged or married.