My wife just had her N-400 interview today and it turned out to be a combo interview, even though we received no notification that it would be a combo interview. I think they have to conduct the I-751 interview as a technicality before doing the N-400, in the event that you are not approved for the N-400 you could still have conditions removed for the green card. But I could be wrong about that.
When my wife was called back at the field office, she went alone because we thought it was only for the N-400. The officer then asked if I was there and that she needed to go get me for the I-751 interview. So it was a good thing that I was there. I had seen on here that there were some cases of having the combo interview without notice, so I was at least somewhat prepared for that possibility. It was a short interview, just some questions about the timeline of our relationship and how we met. Then the I-751 was approved and I was asked to go back to the waiting room. This was at the Nashville field office.