Can someone please explain the difference between "Co" and Jaki"?
Is it not correct to say Czego Widzicie?
No. This verb takes accusative, and the accusative of "co" is.... "co". Take a look at https://en.wiktionary.org/wiki/co#Polish and click the "Declension [show]".
"Czego" is genitive, and so would be more relevant to a negative question.
What indicated second person plural? Why wasn't second person singular possible "Co widzisz?"
Nothing. "Co widzisz?" is equally valid, it's a starred answer. It should have worked.