At least in terms of game mechanics, the difference is clear. Master dueling with a single saber gives you a higher attack bonus and a higher defense. Master two-weapon fighting with either two sabers or a double bladed saber gives you an extra attack and does more damage.
Single saber is better against opponents with a high AC, double sabers or double bladed saber are better against many weaker opponents.
That's the way they balanced in the KOTOR 1 (the only problem was the glitch that made the double bladed saber not get the penalty it was supposed to).
