I don’t think you can get a definite answer to this question. I’ve been watching baseball since the late 1960’s, and there were always a few double switches, although they are much more common today. The strategy is so obvious, however, that I don’t think you can prove that any one manager invented it or “popularized” it. Managers who were ahead of the curve in making a lot of pitching changes, and juggling relief pitchers based on matchups and workload, tended to make more double switches, but Herzog was by no means the first or the only one.