Alma 4:15-17
I think that there are a few details to note from when Alma separated the offices of cheif judge and high priest. First is that his reason was that he saw the afflictions of the humble members being persecuted by prideful members. Second is that he chose to keep the office of high priest knowing that the call from God was more important than the election of men. Third is that he knew that the cheif judge could accomplish much good so he made sure that his successor was a man of the church; in this case, Nephihah was an elder in the church when he became the second chief judge.