You shouldn't need one.
When configured as single channel, each of the 2 controllers can be made to operate as the left or the right channel in mixed mode.
To do this, configure each controller in Mixed Mode and check the "Enable Invert" input. Feed both potentiometers of your joystick to both controllers. Pull the Invert Pin to Ground on one of the two controller. Leave the other Invert Pin open or wire it to 5V.
Cosma