He can use the pioneer speakers, but NON coaxial. Run the one set of wired for
the 6x9 cone. Then install a separate tweeter for the other set of wires.
This is basically how Bi-Amping works with a single channel:
So, you hook the separate speakers up the SAME way.
AND, it's still wiring in Parallel.
Instead of one pair going to the coaxial speaker, and then paralleling from there,
with the woofer and tweeter, they're paralleled right from the amp instead, then
to each separate speaker.
This Bi-Amping the car uses, is one amplifier for the low frequencies and a second
amplifier output for middle and high frequencies. So, you can still wire the separate
speakers to the appropriate pair.