Let’s start with a statement that, hopefully, everyone reading this will agree with: disunity is a major problem among the church. The church is splintered in its expression in spite of being one in Christ. This is a huge problem. However, forcing (or requiring) people to agree with one another about everything does not lead to unity. In the same way, disunity is not caused directly because people disagree with one another.
I think some people despair of unity in the church because they assume it means absolute agreement and conformity. That is fortunately not the case. In some ways I have used that as an excuse over the years. Well, I can't fellowship with those people because they believe A, B and C and of course they are wrong. That is a cheap and unscriptural view that much of the church holds even if they don't come out and admit it. I still struggle with this but I am trying to put aside my pride so that I can more fully experience the fellowship of other believers.
It is not only possible, it is absolutely necessary that we fellowship with people we disagree with on some issues. The only person I agree with 100% of the time is myself and sometimes I make pretty poor company even to me! The church is united by so many things, first and foremost the blood bought adoption by God. If God has chosen us and caused us to be born again and become part of His family, we need to be united with one another regardless of our differences. To do anything else is to tell God that perhaps He made a mistake in bringing those people into His family.