How should I used two MLX90614 sensor on one Arudino Uno given that the sensor has a fixed I2C bus adress?
To get two identical I2C devices on a single I2C bus you need to use an additional I2C device called "I2C-bus multiplexer". Typical 2-channel device is PCA9540B. The multiplexer has its own I2C address, so you will need to add a code that programs it to switch between two "channels", and then use the same routine to access your sensor(s). Here is the typical connection:



As a bonus, the chip can perform level translation if you need one.