With Double I was thinking the Rider and Cadet would combine for Double. The Rider is in control while the cadet gets first person perspective of the action. The Rider would have access to Joker when the cadet is not around.

Or a different route, for instance Bob & Tom are the ones who Transform into Double. They each get a cadet and at any time either one could transform into Double with each other or with their Cadet acting as the other half.