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If we let a paedobaptist into the church, then what will keep that person from becoming an elder? And the church would not want the paedobaptist to become an elder because the elders would then be in disagreement over one of the two ordinances given to the church.

Actually most paedobaptists don't have a problem with baptizing people who have made a profession by immersion. They just see sprinkling and immersion to be both legitimate ways of baptism.
Of course if they did have a big problem, then I doubt they would seek membership or even leadership in the Church.
In my particular Church, people who want baptism and membership are interviewed by the leadership.
When I was in leadership, I made it a priority to make sure everyone being interviewed knew the beliefs of the Church. I also asked them to look over a small booklet stating the doctrine of the Church.
Of course if someone who was a paedobaptist was seeking a leadership position, I would hope that because of the importance of the matter, he would be prevented from doing so.
I do know that if I was still in leadership and I became a paedobaptist, I would step down, both for my own sake and for the rest of the body. But it would not cause me to revoke my membership.

Tom