Yep, all spherical powders are double-based. (Sorry, I automatically call them "spherical" instead of "ball," because Winchester trademarked the term "Ball" after developing the process. At least one magazine attempts to avoid any legal problems by using the generic term spherical, instead of Ball.)
Powder manufacturers often are pretty closed-mouth about real details, though once in a while they slip up and say something other than the basics. Haven't heard about that one, but they might use a coating that helps it burn cooler.
