As Jesus is the cornerstone upon which the Church is built, so are his teachings the unifying cornerstone of Christian doctrine.
The conflict in Matthew 18:15-20 seems to imply that only one side is wrong. In many such situations, each side has caused harm to the other. Is there any application of these principles to this very common situation? It is the same, just doubled. It can be complicated by each side gathering a set of witnesses before taking it to the church or a higher authority if this is a work situation. The obligation of those who are in authority is still to be fair and make an equitable decision. As parents, we are familiar with this kind of situation among our children. It’s really not that much different in the adult world. When both sides are wrong, both sides have acted like gentiles outside the faith. What do we do with gentiles? We preach the gospel to them.