Using the external accessory kit could one iPhone charge another iPhone with a double headed dock connector?
I wouldn't recommend using this cable at all. The iPhone dock connector uses the pin 23 to provide 5V of electric flow. Since you're using a double headed dock connector, the most reasonable technique the manufacturer used is just to split the same cable that comes from the USB head into the four pins each dock connector should at least have to provide power and sync capabilities (or two pins if it only charges the iPhones).

Since the current must at least drop by half on each phone because of the splitting, neither would be properly charged. If both iPhones seem to charge, the least dangerous consequence would be that the charge time will be at least doubled.

Using this technique can (worst case scenario) disrupt the device's PMU. My advice: don't.