Piezoelectric tweeters are a different animal altogether different than your standard dome-tweeters. They use a capacitive element instead of an impedance-based one. You can xover these using resistors to set frequency, and capacitors to set level (L-pad) as the elements are.
If you start with an 8 ohm across the tweeter to simulate an 8 ohm impedance, and use a recommended 22 ohm resistor in series to preclude burnout, you can then use a standard xover like a dome tweeter.
That said- I don't know if those drivers will be friendly and meet in the middle to play nicely.