Why does my custom 2 USB ipad charger circuit not work?

I am trying to use two usb points to double the current to charge an ipad on in-flight usb ports. The cable all up is about 2m long.

I am trying to create something like this: http://www.instructables.com/id/iPad-Dual-USB-Charging-cable/

As in the instructable, it is important that the circuit is wired in parallel (which I thought I did).

I cut the green and white data cables out of the wires, and as per my diagram, got all the black ends (2 from USBs and one from the 30 pin connector) and spliced and soldered them together. I tested it with a multimeter for continuity, and nothing seems to be shorting out. The other thing worth noting is that I extended the cable a little using a two core piece of a Macbook charging cable (I just ran the blacks to the centre core, and the red to the outer core)

Have I done something silly and wired it up in series or something? It won't even charge an iphone!

Thanks!
[IMG]http://media.use.com/images/s_1/d…
Added (1). Thanks very much for that :) Where would the resistors normally be in the charger? I thought they would be located in the charging point you plug into the ipad. (or are they wired into the cable?
>>> Why does my custom 2 USB ipad charger circuit not work?