More than one iPod at a time? You'll have a devil of a time trying to maintain sync. But as long as the audio areas don't overlap it won't be a big deal. But even a few milliseconds of mismatch will be instantly noticeable if you happen to be in listening range of both sources. There's no guarantee that multiple iPods will actually play the same tracks at exactly the same speed.

How about just using a playlist on the iPods?

But what you actually trying to accomplish, there may be another way to approach it.