Using this technique, it's simple for two or more people using the same computer to manage their own iPods, iPhones, and iPads without worrying about interfering with each other's music or settings.
To do this, simply launch iTunes while holding down Option or Shift, as in step 1, to select a given iTunes library. Then connect your iPod, iPhone, or iPad. It will go through the standard syncing process and end at the iPod management screen.
Here, create your sync settings as you normally would. You'll see that you only have the ability to sync items from the library you chose at launch, nothing from the other library/libraries.
An important note about connecting a device that's synced to one library to iTunes using another: Doing this won't allow you to sync anything from the other library. If you do this, you'll have to change the sync settings in iTunes, which would remove all the data from your device and replace it with data from the other iTunes library. Generally this isn't the way to get music from a library other than your own onto your device.
Connecting your device to a library other than your own is OK as long as you don't try to sync anything. Don't change settings or try to sync and you'll be able to charge your device through your computer regardless of what library is running.
