It's generally not a good idea to use the iPad charger for the iPhone, and the iPhone's for the iPad for a couple of reasons.
First, it could either over-supply the battery with power, or under-supply power. Both shortening the life of the battery. Second reason is, the battery is not the only component to worry about. All the circuitry within each device (like the capacitors, resistors, and everything else that make the device tick) are rated for a certain amount of power they need to receive. Too much or too little can shorten the component's life and/or cause them to not work properly.