George Buckbee is best known as the mouthpiece for Expertune, but he also is the president of a little known effort called PIDTutor.
In that persona, George has written and published a fine little book, almost a "chapbook" really, called Mastering Split-Range Control.
This little book is a "back to basics" approach to split range control. It assumes you know little or nothing about how to use two control valves to control one process variable.