One custom board is needed for the project on which the I2C multiplexer, the connectors for the LiPo battery, the gas sensors, the IC2 bus, and the on/off switch are located. The PCB will be mounted in the AR glasses housing.

PCB and stencil for the top layer:

Populated PCB:

To check the function, I wrote a small test program for the Arduino Nano 33 BLE:

#include <Adafruit_SGP30.h>
#include <Wire.h>

Adafruit_SGP30 sgp1; // Define two classes
Adafruit_SGP30 sgp2;

// Function to select I2C BUS
void TCA9548A(uint8_t bus){
  Wire.beginTransmission(0x70);  // TCA9548A address
  Wire.write(1 << bus);          // Send byte to select bus number
  Wire.endTransmission();
}
// Function to print the sensor values
void printValues(Adafruit_SGP30 sgp, int bus) {
  TCA9548A (bus);
  Serial.print("Sensor number on bus");
  Serial.println(bus);
  sgp.IAQmeasure();
  Serial.print("TVOC "); Serial.print(sgp.TVOC); Serial.print(" ppb\t");
  Serial.print("eCO2 "); Serial.print(sgp.eCO2); Serial.println(" ppm");
  Serial.println();
}

void setup() {
  Serial.begin(115200);
  // Start I2C communication with the Multiplexer
  Wire.begin();
  // Init sensor on bus number 0
  TCA9548A(0);
  sgp1.begin();
  // Init sensor on bus number 7
  TCA9548A(7);
  sgp2.begin();
}

void loop() {
  // Print values for sensor 1
  Serial.println("Sensor 1:");
  printValues(sgp1, 0);
  delay(1000);
  // Print values for sensor 2
  Serial.println("Sensor 2:");
  printValues(sgp2, 7);
  delay(1000);
}