Book of Mormon: The Establishment of the Church in Zarahemla
There is an interesting situation in Zarahemla when Alma and his people return. King Benjamin had done this:
Mosiah 6:1 And now, king Benjamin thought it was expedient, after having finished speaking to the people, that he should take the names of all those who had entered into a covenant with God to keep his commandments.
3 And again, it came to pass that when king Benjamin had made an end of all these things, and had consecrated his son Mosiah to be a ruler and a king over his people, and had given him all the charges concerning the kingdom, and also had appointed priests to teach the people, that thereby they might hear and know the commandments of God, and to stir them up in remembrance of the oath which they had made, he dismissed the multitude, and they returned, every one, according to their families, to their own houses.
and King Mosiah continues it.
King Mosiah was both the secular and religious leader at the time—king and high priest; like his father Benjamin and his grandfather Mosiah, and Nephi at the beginning.
Alma’s people want him to be king, but he refuses; he’ll just be their high priest:
Mosiah 23:6 And the people were desirous that Alma should be their king, for he was beloved by his people.
7 But he said unto them: Behold, it is not expedient that we should have a king; for thus saith the Lord: Ye shall not esteem one flesh above another, or one man shall not think himself above another; therefore I say unto you it is not expedient that ye should have a king.
16 And now, Alma was their high priest, he being the founder of their church.
17 And it came to pass that none received authority to preach or to teach except it were by him from God. Therefore he consecrated all their priests and all their teachers; and none were consecrated except they were just men.
18 Therefore they did watch over their people, and did nourish them with things pertaining to righteousness.
When Alma and his church (but possibly not the full law of Moses) come back, they are now under the secular and ecclesiastical authority of King Mosiah:
Mosiah 24:24 And it came to pass that they departed out of the valley, and took their journey into the wilderness.
25 And after they had been in the wilderness twelve days *they arrived in the land of Zarahemla; and king Mosiah did also receive them with joy.
King Mosiah sees Alma and his church, and I imagine he believes it best to allow Alma to be in charge, once more splitting the secular and religious leadership of the Nephites:
Mosiah 25:14 And now it came to pass that when Mosiah had made an end of speaking and reading to the people, he desired that Alma should also speak to the people.
15 And Alma did speak unto them, when they were assembled together in large bodies, and he went from one body to another, preaching unto the people repentance and faith on the Lord.
19 And it came to pass that king Mosiah granted unto Alma that he might establish churches throughout all the land of Zarahemla; and gave him power to ordain priests and teachers over every church.
21 …every church having their priests and their teachers, and every priest preaching the word according as it was delivered to him by the mouth of Alma.
22 And thus, notwithstanding there being many churches they were all one church, yea, even the church of God; for there was nothing preached in all the churches except it were repentance and faith in God.
23 And now there were seven churches in the land of Zarahemla. And it came to pass that whosoever were desirous to take upon them the name of Christ, or of God, they did join the churches of God;
24 And they were called the people of God. And the Lord did pour out his Spirit upon them, and they were blessed, and prospered in the land.
King Mosiah later gives all the religious relics to Alma the younger:
Mosiah 28:20 And now, as I said unto you, that after king Mosiah had done these things, he took the plates of brass, and all the things which he had kept, and conferred them upon Alma, who was the son of Alma; yea, all the records, and also the interpreters, and conferred them upon him, and commanded him that he should keep and preserve them, and also keep a record of the people, handing them down from one generation to another, even as they had been handed down from the time that Lehi left Jerusalem.
But then, Alma the high priest is elected the chief judge:
Mosiah 29:41 And it came to pass that they did appoint judges to rule over them, or to judge them according to the law; and this they did throughout all the land.
42 And it came to pass that Alma was appointed to be the first chief judge, he being also the high priest, his father having conferred the office upon him, and having given him the charge concerning all the affairs of the church.
Shortly after, due to heavy problems in the church, Alma gives up the office of chief judge, and the secular and ecclesiastic powers are once more separated:
Alma 4:15 And now it came to pass that Alma, having seen the afflictions of the humble followers of God, and the persecutions which were heaped upon them by the remainder of his people, and seeing all their inequality, began to be very sorrowful; nevertheless the Spirit of the Lord did not fail him.
16 And he selected a wise man who was among the elders of the church, and gave him power according to the voice of the people, that he might have power to enact laws according to the laws which had been given, and to put them in force according to the wickedness and the crimes of the people.
17 Now this man’s name was Nephihah, and he was appointed chief judge; and he sat in the judgment-seat to judge and to govern the people.
18 Now Alma did not grant unto him the office of being high priest over the church, but he retained the office of high priest unto himself; but he delivered the judgment-seat unto Nephihah.
19 And this he did that he himself might go forth among his people, or among the people of Nephi, that he might preach the word of God unto them…
20 And thus in the commencement of the ninth year of the reign of the judges over the people of Nephi, Alma delivered up the judgment-seat to Nephihah, and confined himself wholly to the high priesthood of the holy order of God…
Later, Alma the Younger offers the religious plates/ records to Nephihah, but he refuses, perhaps because this would signify the reunification of the secular and religious leadership:
Alma 50:37 And it came to pass that in the same year that the people of Nephi had peace restored unto them, that Nephihah, the second chief judge, died, having filled the judgment-seat with perfect uprightness before God.
38 Nevertheless, he had refused Alma to take possession of those records and those things which were esteemed by Alma and his fathers to be most sacred; therefore Alma had conferred them upon his son, Helaman.
(And until the coming of Christ, when it is likely that the government was once more together with the religion.
It then seems to split; after that, secular and ecclesiastic authority seems to come back together at the end—with Mormon and Moroni.)