Different Ipods, multiple libraries: Your music can be new music stored within seperate folders, or music shared with other Itunes libraries. If shared, within the new library, go to add music and select music from your original itunes library. Once you add music, Itunes doesn't care where the music is as long as it's accessible within an available folder, external HD, etc. Also, I guess, if you have enough HD space you could go into your original library and copy music to a new folder and then point your second, third, etc. libraries to the new music folders you've created.
A simpler approach maybe: if you don't need/want to create new libraries because it's a pain switching back and fourth, you could probably just create different playlists for each Ipod. Once connected, within the Ipod settings, you can select "sync with a playlist."