Well, ill try and sum up why dualing at 13 is better then 9, but really, aside from an extra half attack per round, it doesn't matter.
Advantages for dualing at level 9 Max hit points extra 1/2 attack per round from level 7 Extra weapon proficiency point Average recovery time for Kensai skills.
Disadvantages for level 9 Semi low THAC0.
Level 13 advantages An additional 12 hitpoints Extra whole attack per round Two extra weapon profs Higher THAC0 increases hitting ability on most monsters.
Disadvantages Extra 12 hp don't matter Extra half attack from level thirteen is a minor advantage Extra weapon prof hardly matters unless you have True Grandmastery mod installed You will likely have to clear chapter six to regain your Kensai skills if you have a party of 6.
Actually it's less than 12hp and the THAC0 difference is nil. I don't recall exactly but a 20+ thief should have about the same THAC0 as a 13 fighter so it doesn't matter when you dual really. Oh kensai's get +1 to hit every 3 levels so dualling at 13 you get that... big whoop. And the hp well, thieves get at least 1 per level also? That means that a 13 dual only gets 8 more hp... and if thieves get 2 per level (have to check but I think they do) then its only 4hp difference. Proficiencies don't matter that much either... you can put 2 *'s in a weapon at start, then you get an extra at 3, 6, and 9. That's 5 *'s at 9. Getting one extra isn't all that important that you can't wait to put it in after regaining your kensai abilities. In fact getting to 10 thief may in fact be faster than going from 9 to 12 fighter.
Really the only advantage is the extra 1/2 attack, and the extra +1 damage from kensai are really the only major difference.
BUT! Don't knock the 1/2 attack... it's pretty huge.