I'm doing a project with a tandem bike and want to use the rear riders bottom bracket as a jackshaft. I'm having problems finding a dual sprocket. I need a freewheel adapter for a square taper spindle that can accommodate two freewheels. I've attached a picture of what I'm thinking. Also, If it use a Keyed shaft with a two freewheel adapters, how would I adapt the shaft to the bikes bottom bracket. Do they make bearings that can thread into the bottom bracket?