The sound is separated when it is mixed, see:
How is sound from Front speakers changed when installing rears?
When rear speakers are added does this carve away certain aspects of sound from Main speakers?
Does it free up certain stuff being pushed through the main speakers which may improve sound from mains after separating and sending stuff over to the rears?
Maybe another way to ask this is how is the sound separated between all speakers in a 5.1 System?