ATV: Multiple independent audio streams?

each aTV acts as an independent receiver.

from iOS, you can stream to a single location at a time.
you can stream the audio from almost any app. (including while watching videos)
video is up to the app designer, and varies

from iTunes, you can stream audio to multiple locations at a time. (with independent volume control), you can also control this from apple's remote app.
video is to a single location. (you can't stream audio separate from video, so no video on your computer with matching audio from your airplay speakers)

from either, you can change your selection of speakers as you want.

If you're only doing audio, the aTV can be a bit finicky without a monitor attached, you're better off going with an airport express. The older model is usually available refurb for $70, or the current for the same as aTV, $99.
everything above holds true for the express.

also check out airfoil, it allows you to stream audio from programs other than iTunes.

also there are the beginnings of hints that iOS 6 will let the aTV act as an airplay source also instead of just a destination. not sure how that's going to be implemented.

what you want will work.

airplay will allow for multiple sources and destinations on your network. i haven't done exactly what you want to do, but it will work.

The only problem you may run into is available network bandwidth, if you're on a low quality wi-fi network, you may have issues once you start adding multiple streams (or possibly even one stream). but that holds true for any network traffic, not just airplay.

if your router is older, you also may run into issues, but maybe not. sometimes they don't like all the traffic, and need to be reset every so often.