I couldn't find the extra set of joysticks either but after reading the comments above I reopened the box and found both joysticks and a USB3 adapter "thingy" in a small white plastic bag.
Huh, cool idea. thanks for sharing. Are you using this to connect the Mavic controller to your phone? It appears that the cable at 5 feet long would get in the way? I wonder if they make it smaller like 6 inches?