If you don't use the second sensor, likely you don't get readings from it.
mlx.begin(IR1);
mlx2.begin(IR2);
Serial.print("IR2: ");
Serial.print("Ambient = "); Serial.print(mlx2.readAmbientTempC());
Serial.print("*C\tObject = "); Serial.print(mlx2.readObjectTempC()); Serial.println("*C");
temp2 = mlx2.readObjectTempC();
But first you should run the I2C scanner sketch to be sure your addresses are correct.