Try this script in Mojave
iTunes 12.9 will remain in place for Apple to update, start iTunes12.6.app from /Applications/ and create a new library.Code:....
What is going to happen when you use iTunes 12.9 for the same iPhone and then the 12.6? I can keep one of the libraries on a external disk, no problem. But will you get : this iPhone has already been sync with another computer, each time you switch between iTunes ?