If you take two dual 10k linear sliders, wire them backward to each other, take the wipers through voltage followers (to minimize noise from the sliders), take the output of the voltage followers to x&y summing amps - when the sliders are both up you'll get full output from the normally wired slider & zero from the reverse wired slider - as you move the sliders down the normal slider signal is reduced and the reverse signal is increased. Faders intended for audio aren't linear - laser fades should be linear.
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso