@teastain Thanks for the reply. I meant the one titled “Thermal” (without 2) and there is only one module.
Anyway, I solved my initial problem - it was related to not setting up the ext_i2c correctly. I also used a mix of M5Unified and M5AtomS3 libraries because out of the box the FastLED that is referred to from M5Unified does not compile and fails with a static pin value check at compile time. I solved all this (and learned a lot).
Now the only problem I have is: how do I connect the two small M5 modules with a male to male adapter. The grove cable works fine for me but I want a zero length grove male 2 male connection so I can print a 3D case and put both modules into it.