How can I copy music from two iTunes-equipped machines onto one iPod?

You’ll need to choose to manually manage your music (iPod preferences, Music Tab, check Manually Manage Songs and Playlists), then copy music from your iTunes music library to the iPod by dragging songs, albums or playlists onto the iPod icon. When you connect the iPod to another computer, you’ll be able to do the same thing from the iTunes music library on the other computer.