It is generally accepted that firearms are the superior weapon for duelers, so the two most important would be shooting and dodging, followed by aiming. Shooting insures that you deal good damage and dodging covers all bases to some degree - a high dodging skill is better than a small smattering of points in appearance, tactics, reflex, and toughness, for example. The problem with picking just two is that if you pick shooting, you deal good damage but a lack of aim means you don't hit very often, while if you focus on aim instead, you hit often but don't deal very good damage.
As a side note, I would consider choosing another class besides dueler, even if you want to focus on dueling. The bonuses really are not all that great; the increased speed can be replicated by a better steed, the dueling momentum increase is really insignificant (if I recall, it does not even add up to one extra duel per day) and the extra 10% cash is kind of pointless too. I would suggest an adventurer simply because they will be able to make the most money quickly, thereby quickly offsetting the loss in movement speed and 10% extra dueling income, and have the advantage of being able to use lvl 2 hotels anywhere for free, which is a great dueling asset, especially since a dueler won't want to be carrying around a bunch of cash to pay for hotels far away since you will most likely get dueled often yourself.
Oh, and stay clear of soldiers, they have a really good defensive bonus and weapon advantage that will make it hard to duel them unless you are really well equipped / skilled.
Good luck.
-Altsoba
