The problem with piezo is because of their high capacitance they form a high pass filter, and if feeding a load with too low impedance, you lose low end.
You want about 5 to 10 M input impedance. Of course some people use piezos with no preamp. There was a guy who made basses like that.. the piezo and magnetic pickup were wired together passively, and he felt the loading from the mag pickup made a more natural sounding piezo.
Since piezo's don't have resistance, wiring two in parallel wont do anything, as far as lowering the impedance needs. Many acoustic guitars use multiple piezos in parallel.
Here's an even simpler preamp/buffer: