Apple will hold two seperate media events in order to launch the new iPhone and the iPad mini. The reason is this: to ensure a flawless product launch execution. Why? Because Tim Cook is a master at this and this is all part of the Apple experience.
Just remember how every year when Apple announce a new version of the iPhone: Apple Stores are submerged by people, long lines forms and the internet slows to a crawl. I would be very surprised to see two major products announced on Sept 12th. The iPad mini will surely attract big crowds as well as the new iPhone and they will make sure to devote this little device a special media event just before the holidays.