How to control the gain of more than one TDA2030/2050 IC with a single potentiometer?
Don't dink around with the outputs.  Use a volume control at the input.  One potentiometer connected to the inputs of two amplifiers.
Like this:



simulate this circuit &ndash; Schematic created using CircuitLab
Where C1 and C3 are the same as in your diagram.  The gain of the 2030 is fixed - you can't change it.  Just vary the amplitude of the input signal.
Most amplifiers work that way.  The amplifier has a fixed gain, and the volume control varies the amplitude of the signal going into the amplifier.