CN-0350 with multiple Inputs

CN-0350 is designed for a single piezo sensor input. I need to change the design so it is able to sample from two piezo sensors in parallel.

A simple solution would be to duplicate the analog part of the circuit and use a 2-channel ADC (e.g. AD7091R-2 instead of AD7091R). However I would prefer to keep that number of parts low.

The ADC AD7091R-2 exposes the output pin of its input multiplexer and the input pin of the actual adc and thus allows to place analog circuitry between them. So I am guessing if I can just place the whole analog circuit of the CN-0350 behind the input multiplexer and connect the two piezos directly to the ADC input pins. The output signals of the Piezo sensors are quite delicate (small charges) so I am wondering if the ADCs input multiplexer would somehow introduce problems.

If that does not work, would it be sufficient to use one charge amplifier circuit (U1A, R3, R4, C2) per piezo and place the second amplifier stage (U1D, R5, R8, R7, R6) behind the ADC input multiplexer? In that case I would require 5 opamps (-> so one more IC on the board), can't the reference voltage be buffered with a single opamp?

Side-question: whats the purpose of R5?

Best regards