How to create this opamp circuit for mind machine
It sounds like you want to drive the LED roughly in response to the amplitude of the sum of the two audio signals.  Making a signal that is proportional to the amplitude of another is called AM demodulation.  There are various options for doing that.  Such circuits have been around for a long time since they are fundamental to a AM radio receiver.  Yours will run a much lower frequencies, but the concept is the same.

In your case, you also probably want to threshold the signal a little.  In other words, instead of the brightness being proportional to volume, the brightness will be 0 below some volume.  LEDs are diodes, so have a threshold voltage built in to first approximation.  For example, a common green LED drops about 2.1 V when fully lit.  But, the current, and therefore the brightness falls off rapidly as the voltage gets lower.  At 1.5 V it will be so dim you won't notice it on in normal office illumination.

So what I'd do for experimenting is to start by AM detecting the sum of the two audio signals.  Using a opamp amplifier with two pots, allow for adjusting both the gain and offset of this signal.  Then connect that to the LED with a resistor in series.  The resistor should be sized so that the LED current never exceeds its maximum rating even with the largest possible voltage coming out of the amplifier.