As others have astutely noted, Christians are already doing this without knowing it. The problem is that hubris gets in the way. When Christians learn that another Christian differs with them on a secondary issue, it has a tendency to end with division, with separation. Hence our many, many, many, many, many denominations.
The Holy Spirit is stronger than their hubris. If they are willing to humble themselves and submit to the Spirit, He will bring them together. Christians should remember the Scriptures. Jesus prayed that we would be unified, and He stated that others would know we were His disciples by our love for one another. Paul warned about division and being divisive.
I would say, have those Christians sit down and have them talk to each other. They will likely have far more in common than they think. Remind them of the Apostles Creed–it’s a good and ancient creed with its foundation on the teachings of the Apostles, and its focus is on the core doctrines of Christianity, those salvific doctrines.