I think that is a perfectly valid solution. Thanks for sharing.

Thanks
Felix

For now the solution that I've found is to establish a connection once per sensor and end communication.

I'm not sure if is a good practice with I2C communication, but is working for a simple using.
Of course it has a little more delay in the measurements.

  if (!sht3x.begin(&Wire1, SHT3X_I2C_ADDR, 0, 26, 400000U)) {
    Serial.println("Couldn't find SHT3X");
    while (1) delay(1);
  }
  if (sht3x.update()) {
    Serial.print("SHT30 - Temperatura: ");
    Serial.print(sht3x.cTemp);
    Serial.print(" - Umidade: ");
    Serial.println(sht3x.humidity);

    //DISPLAY
    StickCP2.Display.fillRect(110, 30, 100, 30, BLACK);
    StickCP2.Display.setCursor(5, 30);
    StickCP2.Display.setTextColor(BLUE);
    StickCP2.Display.printf("SHT30: %.1f", sht3x.cTemp);
  }

  Wire.end();
  delay(1000);

  if (!sht4x.begin(&Wire, SHT40_I2C_ADDR_44, 32, 33, 400000U)) {
    Serial.println("Couldn't find SHT4X");
    while (1) delay(1);
  }

  sht4x.setPrecision(SHT4X_HIGH_PRECISION);
  sht4x.setHeater(SHT4X_NO_HEATER);

  if (sht4x.update()) {
    Serial.print("SHT40 - Temperatura: ");
    Serial.print(sht4x.cTemp);
    Serial.print(" - Umidade: ");
    Serial.println(sht4x.humidity);

    //DISPLAY
    StickCP2.Display.fillRect(110, 60, 100, 30, BLACK);
    StickCP2.Display.setCursor(5, 60);
    StickCP2.Display.setTextColor(YELLOW);
    StickCP2.Display.printf("SHT40: %.1f", sht4x.cTemp);
  }
  Wire.end();
  delay(1000);

you are corrrect, SHT30 has an ADDR pin which allows to change the I2C address:

The address of the sensor can be changed dynamically during operation by switching the level on the ADDR pin.

However in the ENVIII hat the ADDR pin is connected to GND. See schematic here. This means In order to change the I2C address you'd need to modify your ENVIII on a hardware level.

In contrast, according to its datasheet, SHT40 doesn't have an ADDR pin - the I2C address is predefined and reflected in the part number (A -> 0x44, B -> 0x45)

Products Details
SHT40-AD1B base RH&T accur., 0x44 I2C addr.
SHT40-BD1B base RH&T accur., 0x45 I2C addr.

Thanks
Felix