Unfortunately, what you are asking for won't work. The battery in the phone is not directly connected to the charge input terminals. The phone has an internal circuit to convert the 5Volts from the connector to the 3 to 4.2 volts needed to charge the internal battery. But that circuit doesn't work in reverse. So, even if you had such a cable, you could not take power from one phone to charge a second phone.
Looks like it puts a small output from two phones in series to try and charge one.
You need 5 volts DC to charge a phone up to 500ma.
There maybe 3.3 volts at a low current available at the connectors witch when put together will give you 6.6 volts at some current to charge a phone.
It may take a long time and kill the batteries on the other phones to charge up one.
Its just not practical.
Notice the product in your link is not available yet and may never be!
Power might be available at the usb socket, or maybe even the memory socket, and you could use a switching reg to step up or down the volatge, but the amount of power available it'd probably be fairly useless.
One way you could do it is to make a charger that runs from a phone battery, you'd have to pull the battery from the phone to use it, which is also not a great deal of use.