I've wondered about this too. Since I don't have the option of separating them, I decided to twist the two pairs together with 3 twists per foot - nothing scientific. I don't know if this is a good alternative, but fortunately I don't have golden audiophile ears so I doubt I could discern any difference anyway.
Seperate bi wire cables?
For best results do you need to keep bi wire runs seperated from each other by an inch or so?