1. Open iTunes.
2. Select the source iPod from the Devices list in the left sidebar.
3. Click on the Summary tab.
4. In the Options section, check the Enable disk use box.
5. Click on the Apply button.
6. Select the target iPod from the Devices list.
7. Click on the Music tab.
8. In the Sync Music section, select the "Manually manage music and videos" option.
9. Click on the Sync button.
If you only have one iPod connected to your computer:
1. Connect both iPods to your computer.
2. Open Finder.
3. Select the source iPod from the Devices section in the left sidebar.
4. Open the Music folder.
5. Select the music files that you want to transfer to the target iPod.
6. Drag and drop the music files to the target iPod in the left sidebar.