(I searched for a related discussion but couldn't find one, so if this has already been discussed, please point me in the right direction.)

I'm about to upgrade to 7.1 speakers and was wondering how the rear channels of a 5.1 soundtrack, such as DD, DTS, etc., are divided up between the four rear channels. Do they have two different soundtracks encoded on the disk?

Also, if DVD's have two different soundtracks, one for 5.1 and 7.1, what about SACD?