You can actually set up a seperate library for second and additional ipods, this works well if you have different tastes in music or want a different type of genre on each ipod.......saves having to keep ticking and reselecting you songs each time you want to sync.
That was the easy bit, now unfotunately I cant remember how you do it......should be a link on the apple help website...If I remember you hold down one o two key when you start up itunes and you can then create a new library.
Hope that helps