absolutely you can use a dual potentiometer for this, pick one with a linear (aka B) taper and the same resistance.

but you can probably modify them by interconnecting them so that the second unit acts as the opposite of the first.

Here blue lines are wires (you'll need to scrape away some of the green to solder to the copper under it) and the red lines are cuts you'll need to make in the copper of the second board.

also connect the power and ground on both units.

absolutely you can use a dual potentiometer for this, pick one with a linear (aka B) taper and the same resistance.

but you can probably modify them by interconnecting them so that the second unit acts as the opposite of the first.

Here blue lines are wires (you'll need to scrape away some of the green to solder to the copper under it) and the red lines are cuts you'll need to make in the copper of the second board.

also connect the power and ground on both units.

However seeing as you want dark on the middle setting the dual potentiometer is probably the easiest way.

^{simulate this circuit – Schematic created using CircuitLab}

Here out is out is the centre potentiometer terminal, 0V is the negative supply and +5 is the output of the L7805 regulator chip. there will be two resistor associated with the existing potentiometer, one of them will be from 0V to the potentiometer and the other from +5 to the potentiometer, the 0V one will can be bypassed (or replaced with a piece of wire) the +5 one can be swapped for a 27K (or sommething approximately half the resistance of the potentiometer - if your replacement potentiometer has twice the resistance of the original this part will probably not need to be changed)

absolutely you can use a dual potentiometer for this, pick one with a linear (aka B) taper and the same resistance.

absolutely you can use a dual potentiometer for this, pick one with a linear (aka B) taper and the same resistance.

but you can probably modify them by interconnecting them so that the second unit acts as the opposite of the first.

Here blue lines are wires (you'll need to scrape away some of the green to solder to the copper under it) and the red lines are cuts you'll need to make in the copper of the second board.

also connect the power and ground on both units.