Hello PCF,
You would do it by putting it on a separate Division object, and setting up couplers as desired (using Coupler.CouplerCodeFromWhichToCopyState when applicable), similar to the way the 'Swell Reeds on Choir' coupler is implemented in ExampleCustomOrgan2. The 'stops' that control it (on either of the two real divisions) would actually instead be implemented as couplers.