this is BS. There is no competition because no one is going to buy both regardless. If they release the 4.7" first people will buy it (for less money) and then will just be pissed off that they can't get the 5.5" when it comes out because they aren't eligible for a hardware upgrade anymore
Believe it or not there are phone fanatics who buy multiple handsets every year. And if Apple can be seen to be launching new iPhones every six months, that breathes new life into the product twice a year versus the current launch cycle of offering new phones once every 12 months. That's some positive justification I can see for separate launches.