Backup second iPhone to my PC How can I backup a second iPhone 4S to my PC? Both are backed up in iCloud but only one in iTunes store.
When an iPhone is backing up to iCloud, iTunes doesn't automatically do a backup of the phone when it syncs.

You can, however, manually tell iTunes to back up the iPhone. When you connect the iPhone to iTunes, and you click on it and it opens to the summary page (where it tells you the name of the phone, and gives you the option to restore it and see home much space on it is used), there is a Backup button in the right side of that page.

Check out Apple's "HT5262: Choosing an iOS backup method" on Apple Knowledgebase. It talks about the benefits of iCloud backup, the benefits of iTunes backup, and then it talks about how to do both together.