Contrary to a flurry of reports stating otherwise, both 4.7 and 5.5-inch iPhone 6 models will in fact hit store shelves at the same time, on Friday, September 19, ten days following tomorrow's announcements. This information reported by the French blog iGen.fr [Google Translate] came from the same source who in the past accurately predicted other Apple launches.
It remains to be seen whether this is just an educated guess or the real launch date. The rumor-mill's been saying for months that the 5.5-incher would hit later due to alleged manufacturing challenges. For what it's worth, Apple typically starts selling new iPhones 7-10 days following the introduction, or the Friday the week after the announcement.
For instance, the iPhone 5s/5c went on sale on September 20, 2013 following its September 10 unveiling. And before it, the iPhone 5 reached retail stores on September 21, 2012 after it had been introduced on September 12.