If you are buying a Dually Halter for use loading horses Intelligent Horsemanship recommends the following DVD for help with correct technique and pointers to common problems and solutions:

 Horsemanship Essentials DVD - 'Loading the Less Easy Horse' 

Absolutely essential when working with reluctant loaders is a decent length of line. This ensures that you are able to move the horses feets without him ever feeling that he is able to move yours:

20ft Lanky Lead Line