Separating iTunes into multiple accounts

Probably the easiest way to do this...

Have your sons each create a playlist each (*Tom's playlist* and *Bobby's playlist*)and then add all the songs they want to their playlist.
Create a folder on the desktop, select all the songs in one playlist and drag them to the folder on the desktop.
Then copy this folder to their own computer and drop them into the *Automatically add to iTunes* folder inside the iTunes folder.

Their computers now have all the music they want.

To clean up your iTunes, select all the songs from *Tom's playlist* and drag them to the Bobby's playlist, then delete *Tom's playlist*.
*Bobby's playlist* will now contain all the songs from both of their playlists.

Go through *Bobby's playlist* and delete songs you want to keep.
*Bobby's playlist* _should contain only songs you want to remove from your computer_.
Once *Bobby's playlist* contains _only songs you want to remove from your computer_, select all the songs in *Bobby's playlist* then hold Shift and press Delete.
This will remove them from your iTunes library and delete them from your computer.

In addition to the steps described by Chris, you will need to determine if any of the songs are DRM-encumbered tracks that were purchased from the iTunes Store. If so, each of your sons' computers will need to be authorized to the account(s) from which the songs were originally purchased. (This will also apply to all future computers they may acquire, unless you upgrade the tracks to iTunes Plus.)

THey can create their own libraries then authorize the computer with your iTunes account. Then plug in their iPhone and File -> Transfer purchases to get the app into their library.