Can two switches play off one nintendo account at once?

Also worth knowing: each kid should have their own Nintendo Account on their own console, joined to your family group. That way their saves are their own and parental controls work per child. If both are playing off your account, saves and play time end up mixed together.

Different games at the same time, yes. The same game at the same time, no. Nintendo moved digital games to virtual game cards, where each game is loaded on one console at a time. Put some on each kid's Switch and they can both play, they just can't play the same one at once. If they're in your family group you can also lend a game to the other console. Moving a game back and forth needs both consoles online now and then, so don't expect to swap it instantly every time.