This can't be true, otherwise you'd learn all languages and it would take, practically, no longer than learning one.
At school if a child is learning 2 languages (L2, L3) then do you think they can do that with no more time commitment than learning one additional language - if you can explain how then you can revolutionise language learning .. all schools can teach the kids 20 languages within the lesson time they currently only manage one.
In practice what's happening is you're teaching in an open manner, it's not directed learning but it is occupying more time, and mental processing, than learning only one vocabulary and one grammar. There are probably a lot of efficiency gains, but if exposure and use is at all important to language learning and proficiency (they are according to most comments here) then you need more time.
When that time is folded in to social times or family time it appears to be "free". The problem comes when people who have learnt/taught this way get to claim kids in school can learn an additional language because it doesn't take any time (and so, the logic goes, can't negatively impact other learning).