Seems to me that the general benefits of biwiring are to be had by separating the WOOFER from the TWEETER, rather than the positive and the negative. I think the major benefit of properly designed biwire speakers is to separate the high current = high em fields of the woofer's crossover, connections, and wire from the lower power path of the tweeter. While I'm sure the other method would work fine (one wire for both +s and one for both -s), I would bet the "traditional" method would give the better end result. Admittedly, I have never tried the alternative.
All of this assumes of course, that your speakers' design truly benefits from biwire. I have had some speakers that did not benefit much at all, and some that only came to life when biwired. So what the heck try it both ways.
The MOST IMPORTANT thing about Cardas cables is that they need to SETTLE. After you dork them around to make the connections, they will take 2-3 days to settle in to the sound they are actually designed for. A lot of cables exhibit this, though Cardas seems to be particularly sensitive. (Albert has mentioned Purist Audio being extremely sensitive to being "dorked around" as well)