There are basically two ways of using multiple iPods on a computer and these involve:
a) Sharing a single iTunes library and user account.
b) Creating multiple user accounts and having separate libraries.
Sharing a Library
If you want to share the one library, you can set either or all of the iPods so that they only get updated with only certain playlists (you can update from more than one if you wish):
Loading songs onto iPod automatically - Windows
Choosing the update option "Sync Music - Selected playlists" allows you to create a playlist specifically for the iPod and drag the tracks you want into it. If you tire of the list and want to change it, you just add or remove the songs you don't want. The ones you take out out remain in the library to be used some other time if you choose. You can read more about playlists at these links:
iTunes: Creating playlists of your favorite songs
How to create a Smart Playlist with iTunes
Or you can choose to update any or all of the iPods manually and just drag whatever content you want to them:
Managing content manually on iPod
Separate User Accounts
Another option is to create a separate User account for each person on your PC or Mac. Different accounts by definition would give you completely separate libraries. Each account has it's own iTunes folder, Library and iTunes Music folder and you load it with CDs etc just as you did with your original one. The iPod can be set to update however the owner chooses, sync all, manual or sync specific playlists
