Originally Posted by TechRep

One way to do it would be to set one (or both) iPods to update only the songs on a given playlist (so you would have a playlist called "StarG's iPod" or something similar). You do this by plugging in the iPod, and then going to iTunes' preferences window. Hit "iPod preferences" and then switch to "Automatically Update Selected Playlist Only", and choose your playlist. Then, when you add a new song to your library just add it to the playlist and presto, it will only copy to that iPod.