That's good.
Did you get that original example to prove that the thermos used only one at a time work properly? That is before you added to the code and hardware?
Can you try simply assigned a value like 42.0 to the second thermo, that is leave it in your test code but do not call it:
//replace
double co = thermocoupleO.readCelsius();
//with
double co = 42.0;
Also, can you power your hardware other than through the UNO? It is not meant to be able to supply much power externally; it is best just to plan on not using it. A bit of hanging out will show that power problems pop up perennially in these parts.
a7