I think you have to do it semi-manually: sorty by Date Added (or use a Smart Playlist), select the songs in question, and drag them from iTunes onto the other machine. Then go to the other machine and drag the songs into iTunes.
In my experience you can only sync ONE computer per data-type per iPod. So my iPhone (in my case) syncs music, apps and "other info" (contacts/calenders/email accounts) with one machine, and it syncs photos with another machine. But it can't sync music with two machines, or photos with two machines.
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