Use playlists ... thats the only "EASY" way. Or use the GROUPING tag and then use "Smart Playlists"
If you set all of your own songs to have "Dad's Music" in the Grouping tag you can then create two smart playlists:
The first will probably look something like this:
http://i120.photobucket.com/albums/o168/VixenofVenus/smartplaylist1.jpg
The second will just change "is" to "is not"
So then both iPods to sync only selected playlists and make sure that is one of the playlists selected for each iPod (yours is the "is", his is the "is not"). You can also select any other playlists or whole folders of playlists, but as long as you keep your playlists and his separate, it will work fine.
I use a similar method for my own two iPods.
