Using different gauge wire for biwiring

It depends how cable is built. From manufacturer's point of view skin effect in copper starts at 20kHz at about gauge 18. Using stranded wire is not the best thing since current jumps constantly from strand to strand (to get on the outside) going thru impurities of the surface (oxides). Using isolated strands solves the problem by increasing surface area but strands are still in common magnetic field of each other (that adds). Placing strands on oversize tubes (Audioquest, Acoustic Zen) or making flat cables puts each strand in magnetic field of neighboring wires only (mostly).

I don't know if skin effect is audible but I know that cable manufacturers believe it is and come-up with such exotic schemes. My Acoustic Zen Satori Shotgun uses same gauges for woofer and the tweeter (many isolated strands of solid wire about 22ga on a big hollow tube plus one stranded wire with about 10 strands of very thin wire) but it is recommended mostly for tweeter+midrange on one and woofer on the other (or 4 way systems). For two way systems they recommend another cable where tweeter and mid/woofer have different gauges - Hologram Bi-wire.

Some info is available at Audioqest website: http://www.audioquest.com/pdfs/aq_cable_theory.pdf

and Acoutic Zen website:
http://www.acousticzen.com/white_paper.html

I don't question their findings since I've never designed any cable.

Rhljazz, quite simply, it sounds better.

Anyone like the Anticable in biwire?

I went thru a rather exhausting speaker cable search going from name brand cabling to DIY and all manner of mix and matching in guage and composition.

For example I used a combo of solid core copper and solid silver of different guages, one for the top one for the bottom.
To make a long story short, some music seemed to favour one type of set up,and sometimes the thicker guage on the top end made for a more full midrange type of sound.
At other times, this wasn't a good thing.
As far as jumpers go, the stock ones weren't very good, and no jumper set up was as good as two separate runs of wire.
I am in agreement with keeping both runs exactly the same.

In my case the system sounds more coherent.

I agree with Lacee. Same cable for tweets and woofs, and same lenghts as well.