Are the iPad 2 30-pin cable and the iPod touch cable different?
The USB-to-30-pin-dock cables are electrically identical. All of them can carry enough charging current for both devices. The same is not true for the power sources as the iPad gets a 10W supply and the rest 5W.

What power source are you using with the cables? iPads need more power than most USB ports offer to charge with the screen on.