Look at the output of the piezo sensors with an oscilloscope if possible. You will find that they emit an AC pulse when flexed and restored. The pulse is pretty short (typically), so you will have to get both piezos to flex and restore in phase if you want the voltage to add constructively.
There are dedicated energy harvesting buck-boost converters (not sure if they are appropriate for piezos though) that take whatever voltage is generated (1-6V for instance) and convert that voltage to a specific, fixed output voltage (4.2V for Lion charging for example). Note that power is conserved during this conversion so if the voltage is increased, the current will be decreased and visa-versa.
I believe there are also idea-diode full bridge controllers that can be useful in converting the AC output signal into the DC equivalent with very low losses (using diodes would loose a significant amount of your power as there would be two 0.5V diode drops in the bridge reducing your 3v to 2v with no change in current).
If you are trying to charge a cell phone with piezo generators, figure out the power each piezo is rated to generate (average watts) and figure out how many you would need to generate the 11W or so an iphone wants for charging (don't forget to take duty cycle into account). If the project is still practical, then you can start designing the electronics to actually harvest that energy which will be a challenge to do inexpensively.
