1) If you live in an area where carrying a knife is part of survival maybe you should invest the time and money in throwing knives. While this can be a hard skill to learn it can give you a definite advantage.
2) In a knife fight its all about feinting and lunging. If they other fighter is holding the knife in the classical butcher hold, a quick feint to their opposite hand and then a jab at their throat can end a fight quickly, this will usually only work on amateurs.
3) If you're an amateur, stay away from a knife in each hand, it'll only confuse you.