@wvmarle, I think I was pretty clear stating that I want "to have one board both acting as a Transmitter and reading temperatures from the thermocouple" and that my problem is that "the nRF24L01 interacts with the MAX6675 in a way that makes the MAX6675 constantly give a reading of 0.00degC".
@Robin2, I tried swapping the lines and doen's make any difference. Then I commented the initialization of the radio and the related functions in the setup and loop, and the MAX6675 started giving correct values, so it probably has something to do with what you said.