Oh I think I get it now - to play the 2v2 Ranked matches, you have to have a guest player on the same console.
It's not 2v2 on 4 different consoles, it's two consoles but each with a guest - you have to go to "Settings>Add Guest User" in the Quick Match menu, then press Start on the second controller, after you do this, the 2v2 Ranked Match option will no longer be greyed out.
You can play online 2v2 (4 consoles) in the lobbies, or in Friendly matches, but neither are ranked.