"As long as we're on the same page" = "as long as we're thinking the same" or "as long as we understand each other"

It means in agreement, in harmony, together. A common use might occur when several people, who represent the same organization, are agreeing to present the same message. "Before we go to present our proposal, let's just make sure we are all on the same page, so that you won't be telling them anything different from what I've told them." I think it is a metaphor referring to a church congregation, which may be asked to take out a book and sing a hymn. One can imagine someone saying "before we start singing, let's make sure we are all on the same page." You will occasionally hear the phrase "let's make sure we're all singing from the same hymn book."

"On the same page" basically means that more than one person have the same idea or are talking about the same thing or are sharing the same concept. Then, in this case, I think that it means something like: "If we can stay on the same page". I'm not a native speaker. Probably, they have a better answer than mine. Anyway, I think that it can help you.