Using different gauge wire for biwiring

Using smaller gauge wire for the tweeter is not uncommon. I can think of a couple of manufacturers who build biwire cables that way. Dh labs Silver Sonic Q-10 uses 12 gauge for woofer and 14 gauge for tweeter.

You can use different gauge wire for highs and lows... thicker wire goes to the woofers.

Most internal speaker wiring is done that way. Also, some manufactured cables such as AZ Double Barrel use the same strategy (6awg and 10awg).

I've compared using smaller guage cables on top vs. using the same cable for top and bottom. The most phase correct result comes from using the same for the top and bottom.

Inside the speaker, the mfg may use different size wires for balancing the sound but, in the end, the speaker is designed to be hooked up with a single wire speaker cable and sound correct. If you slightly reduce the amount of energy to the top by using a smaller ga cable you are disrupting the balance of the speaker the way it was designed.

Good luck..