I don't think you are actually drawing twice the current. I think what is happening is that the iPhone is stealing some of the current from the iPad, so the iPhone will charge at normal speed and the iPad will charge at a slower speed.
phone 6, 6 plus and 6s* and 6s plus* ( * = el capitan required ) asks for the same charging current as an ipad.
looking at the picture, what i would do is go into about this mac. go to system report. click on usb, and then click on the device that is connected. you will see how much power the 2 way adapter is asking for. and then you would just divide this by 2 evenly.
you should see:
current available ( ma) : xxx
current required ( ma ) xxx
Extra Operating Current (ma) : xxx
add extra operating to current available and you get the total current the usb adapter is getting.
then this current is divided up and shared among the two devices
its my guess that its probably asking for 2100 ma? how it splits it , i have no idea.
it could be just like a dumb 2 way cable and the mac is actualy charging both devices at their top speed. i am really not sure. i think its probably like a usb hub or a usb bridge and it takes whatever power your giving it, and it divides it up, instead of your mac dividing the current
this is why macs have system report
if the ipad + iPhone are both in system report , then just add the current together.
your mac will tell you exactly how much current every usb device uses
if anyone wants to ponder the idea, of how much an iPod touch 6 gen will need or if the ipad charger will charge an iPhone 5 any faster. you can use system report to check every single iOS device you have
