Leading one horse while riding another, how to teach ponying

Episode 81: Today's question is about teaching horses to pony or the art of leading one horse while riding another. Ponying can be a great way to exercise two horses at once as well as a way to expose a younger or less experienced horse to more of the world without riding. 
This podcast lists things to consider regarding the horse you will be riding as well as the horse you will be leading or ponying. 
It also discusses controlling your environment and situations you should prepare for such as narrow trails, barking dogs, etc