The CS for each ADC is different, so 2 ADCs will not be selected together. I'm also thinking about the grounding. I noticed the difference between the ADC ground and the base board ground , and the value changed a little bit when another ADC was plugged in.
I also tried to check with a scope, but it seems that the scope I used will influence the reading. Is it because the scope not good enough? It's a digital one, the proble of which has a resistence of 10M(ohm).