Electronic Circuit to Voltage control pan of stereo audio? (Modular Synth)
Concentrate on one of the stereo outputs first, say left. You want, at one extreme for it be be the left input signal whilst at the other extreme you want it to be the right input signal.

So, you need two voltage controlled amplifiers (VCAs) that can attenuate a signal to nearly nothing and those two VCAs need to operate oppositely to each other such that when you sum the two VCA outputs you’ll get left input at one extreme, pure mono in the middle and right at the other extreme.

Ok so far?

Then you need to repeat for the right output channel.

As regards a VCA that is effective, you could use an analogue gate being activated at high frequency (maybe 100 kHz or more) by a pulse width modulated square wave. The duty cycle modulates (turns on and off) the audio input and, because high switching speeds are used, the output amplitude is effectively controlled. This is an example of a triangle wave being switched on and off by a square wave (50% duty). The "gated" signal is then low pass filtered to remove "above audio artefacts" and the resulting amplitude is 50% of the original: -



Here's an example of a PWM circuit using Linear Technologies LTC6992: -



There are other types of VCA but to go into them is really another question.

So draw out (block diagram) what I’ve described and see if it still makes sense to you.

Pictures also covered in this and this answer.