It's never been revealed or confirmed. All notions of local wireless play has one screen per person.
Yeah that´s what thought. Reading my initial post I should have worded it better though, what I really want is local splitscreen in 1 Switch. And coming back to the main topic, until Splatoon doesn´t have that for 4 players MK will remain the king of local play. Besides, from my own experience it isn´t a game as easy to get into as MK, where you only need 2 buttons and 2 directions; Splatoon requires moving in one direction while aiming at a different one and 6+ buttons (Jump, Shoot, Turn to skid, Secondary weapon, Special, Show the map, Jump to a teammate...). That is a very big barrier of entry for a lot of people.
Don´t get me wrong, I´m a HUGE Splatoon fan, it´s my most played WiiU game by far, that´s why I´m so disappointed in it not having local splitscreen. Playing this with friends next to you would be awesome.