I have no doubt in my mind that the two iPhone models will launch at different times. Rumours about delays for the 5.5 inch phone have been swirling for months. Launching a month apart, whilst slightly inconvenient, has many upsides for PR. Apple can control the news cycle twice.
More importantly, it is paramount that Apple gets its new phone out as soon as possible. I think everyone agrees that the 4.7 inch model will be more popular, so it would be bad business to delay the launch to launch both devices simultaneously.
What I can’t justify is an August launch, however. The last product Apple released in August was a minor Mac Pro update in 2010. August isn’t a good time to launch products. A lot of people in the tech industry are away on holiday — the press isn’t set up for covering big launches in August.
I would expect a similar timeframe as last year. Apple would announce both phones in early September, with a mid September release for the 4.7 inch model. The launch of the larger model would then follow a couple of weeks later, probably in early October.