Not sure about Mac, but I am running Vista and set up 2 playlists, one for my Ipod and the another for my sons. When I set up the Ipods it asked which playlist I wanted to synchronise with and has remembered them.
Everytime I plug each Ipod in it recognises it and synchronises with the relevant playlist.
If you just plug them in they'll allow you to sync on whatever basis you want. Each iPod will have its own settings to you can sync parts of your library to each, or everything to both, or whatever - space allowing. Or you can have totally individual libraries on either the same account or different ones.