After he became Chief Justice of the Supreme Court, he had already been President. The Branches are theoretically co-equal. Presidents are often referred to as President even after leaving office for formal purposes.
Suppose I was introducing him at a dinner in 1925, is it "Mr President Chief Justice Taft", or "Mr Chief Justice President Taft"?
Did CJ take precedence because he held that office in 1925? What about in the month prior to his death after he resigned from the court? Did newspaper announcements call him CJ, P, or P, CJ?
If you were addressing him formally, he would be either "Mr Chief Justice Taft" or simply "Chief Justice Taft" as that was his current title. Introducing him would be, "Mr. William Taft, Chief Justice of the United States."
It is important to remember that while it is sometimes said, "The title of President is for life," this is merely a custom of contemporary etiquette. Nothing in the US constitution, the law, or anything inherent about the office itself stipulates that former presidents retain their title. It is merely currently fashionable in the United States to address individuals by their last highest title as a courtesy (rather like continuing to call someone "General" or "Judge" even after retirement, as is the case with James Mattis. It is easy to find many references to "General Mattis" despite him retiring in 2013).
In obituaries for William Howard Taft from 1930 you can find him referred to as "the former President and chief justice" or as "the 27th President of the United States." The New York Times headline blared, "EX-PRESIDENT TAFT DIES AT CAPITAL" and later referred to him as "the Former Executive and Chief Justice." However he is never referred to as "President Taft." Even today, when the custom of referring to people by their former title is more prevalent, it's common to see references to "former President [name]" in news media - probably to avoid confusion.