Is playing tandem considered account sharing and is it bannable?

Yes, but, who would even know?

Considering they literally had pros doing this during an event, I'm going to say no.

I mean someone is playing on your account while you are also playing it as well. But the system couldnt possible know unless you are outright stating it which probably would label you as account sharing.

I think account sharing mostly is against ToS to prevent boosting not so much to prevent this sort of thing. If you can do this with enough skill that it isn't causing your teams to lose every game no one will really know or care.

I don't know if that's what Riot intended when they defined account sharing? Sounds like something people would do on twitch or something you post to youtube

lesser of account sharing, probably more of "trolling" and can be reported as unsportsmanlike conduct.

Well Brofresco did it and he didn't get banned so i would assume it's ok. But he was probably playing a normal or a custom game.