Hi. I was just about to do the same thing: allowing editors to search custom posts not only by titles, but also by a secondary title field.
However I am unable to make this work with the solution PeterSeb provided (also adding Join filter from the article). Looks like search for fields is completely ignored. Working if the query is run separately as a test, but not working here.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users