Sometimes only a joystick or a gamepad is not enough for a use case but other USB devices like rudder pedals and a headtracker should be mixed into the PPM signal.
If the devices are recognized as Windows joysticks/gamepads, then Joystick Gremlin in conjunction with vJoy provides the ability to combine devices into one or more new virtual devices.
The Joystick2PPM – applications Joystick2PPM, Joystick2PPM4Arduino, USBtoPPMforCompuFly and JoystickRC4Diddyborg offer all connected (also virtual) joysticks then at program start as input device and enable by virtualization the assignment of the axes of the different USB devices to the PPM channels.
The installation of Joystick Gremlin is described in the manual and was possible without any problems. Then you configure virtual devices with the desired assignment of axes and keys.
If one of the Joystick2PPM apps finds more than one device (no matter if physical or virtual) at startup, the user is offered a selection with all found devices at program startup. 
If no selection is made by the user, then the first joystick in the list is automatically used.