Syncing your iPod with a secondary computer and iTunes account is simple. All you need to do is adjust the default settings of the iPod and perform a little manual labour. Manually managing the music on your iPod is nearly as easy and pain-free as the automatic sync option but gives you the freedom to add music from multiple sources without deleting or overwriting any music already on the device.
An iPod can only be synced with one home computer. By "sync," this means that you will automatically sync where you connect your iPod to your iPod dock; iTunes automatically updates all the new music and playlist adjustments to your device while you wait until the process is done. This can only be done on one computer. Sync does not necessarily mean "automatic;" sync could simply mean copying music and videos from a computer to the device to match another user's content. This can be done, but you do it manually.
Setting your iPod to manual sync means that when you connect your iPod to your computer, iTunes does nothing. Without user direction, iTunes has no idea what music or videos to put on your iPod. In manual mode, you must drag and drop the files you wish to add to your device to begin the sync. This may sound tedious, but it is the same as an automatic sync, minus the automation.
How It is Done: Home Computer
Connect your iPod to your "home" computer, the computer that you use your iPod the most on. Within iTunes, select your iPod from the sidebar and click the "Summary" tab. You will notice just above the graphic displaying the available space that there are a few check boxes--one for "Automatically Sync with iTunes" and one for "Manually Manage Music." Check the box next to "Manually Manage Music" (and uncheck the automatic option if does not do so itself). If there are additional songs on your home computer that you wish to copy over to the device, grab the items and drop them on the iPod in the sidebar. Watch it sync. This is where your playlists come in handy. When you create the playlists ahead of time, you can simply drag and drop the playlist onto your iPod in the sidebar.
How It is Done: Second iTunes Account
You are now ready to add your friend's iTunes to your device. Disconnect from your computer and connect to the second computer and fire up iTunes. Your iPod will appear in the Devices sidebar. Drag the desired playlist (or files) onto the iPod, drop them and watch this second iTunes sync the files to your device. This adds the new music files to your device while keeping the original files completely intact. The automatic sync option would completely erase and replace your files. If your friend has iTunes' copy-protected files, you will need her password. Luckily, you can play back protected files on five devices. Also, iTunes no longer supplies protected files.
Mac vs. PC
A Windows-formatted iPod (a PC is the home computer) will work on both a Mac and PC computer, but a Mac-formatted iPod will not work on Windows. Keep this in mind if you plan on syncing from two different operating systems. The Windows-formatted iPod has the maximum compatibility.