joystick mechanics

[Paul Perry]

there is a mechanical way(?differential gears) to convert
a joystick motion to two separate rotary motions.... so that
someone with a joystick can use any pots they want....
but i don't know the way!!!!! where is the Radiation Laboratory
'Computing Linkages' volume when you want it? ;)

seriously, can I make this with standard mech parts?

paul perry melbourne australia