I can't find anything that specifically defines what constitutes a majority in judicial cases. It may well just be the result of courts moving from individual judges making their opinions to judges either agreeing or disagreeing with the chief justice's opinion. Article III of the Constitution is the section that defines the judicial branch of the government. Since that Article doesn't define what constitutes a majority opinion, a majority opinion should be able to be defined by Congress without running afoul of the Constitution.
Justices are appointed by the President with approval by the Senate. I suspect that when you have a Democrat President and a Democrat majority in the Senate, they are going to want to preserve simple majority opinions when the court is staffed by left-leaning judges. If the court was filled with right-leaning judges, the current Senate and Executive would probably want to hamstring those judges and might be more apt to support such a change. So, to do what you are proposing, you would have to pretty much have a majority of judges that leaned opposite of the entire Legislative and Executive branches. I think only then would an acting body of Congress be willing to restrict the Judicial branch.
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