The shielded wire is a must but unless those wires are twisted together you don't really have cross-talk issues. The voltages you report don't make sense unless signal and ground wires are transposed on your drawing. I seriously doubt that you connected the piezo's together and then twisted all the wires together. the "normal" ones I an used to have shielded cables from the factory (Piezo's for car burglar alarm's) and those don't cross talk at all. Spacings of a half inch or 12 - 3 mm would drop the levels by 10 db and I don't see in the data anything that resembles cross-talk in your measurements at all.
Doc