Is there any official documentation stating whether you can play out of the headphones and speaker at the same time?

I have spent some time researching this, and it looks like there is only one post on SO where someone claims to have done this, however their code does not work/ is incomplete. They have some strucutres that are not defined and I cannot tell what it is supposed to be. I am looking for some concrete evidence that states one way or the other. There are several people claiming to be able to do this, however it does not work. Does anyone know of any apps that can do this ( avaudiosessioncategorymultiroute seems to be for allowing you to route to multiple place, yes... but does it let you do the speakers and headphones simultaneously, is the question)?