The idea is that you can add multiple (up to 4, but you could extend easily by modifying the code just a little bit) MAX31865 on the same project. Wire the SDI, SDO and CLK of all the MAXs to your HW ...
Adafruit_MAX31865 thermo = Adafruit_MAX31865(MAX31865_CS, MAX31865_MOSI, MAX31865_MISO, MAX31865_CLK); ...