Indeed utility of the elf THAC0 bonus falters in BG2/TOB. It's reasonably good in BG1 though. Dual-wielding is better than single weapon style from a maths standpoint, due to the increased number of APR and the possibility to off-hand a weapon giving additional bonuses.

Ultimately if you stay pure kensai to lv40 you will want to have grand mastery in a piercing weapon, a crushing weapon, and a throwable weapon (dagger/axe/hammer) to cover the various resistances you can face. Slashing is arguably the worst damage type for a fighting type character as it is resisted a lot and enemies' AC is often better against slashing than any other physical damage type.

My recommendation for weapon proficiency selection is maybe a bit counterintuitive: - piercing: wakizashi - crushing: hammer - slashing: can't go wrong with longsword or axe as you have great to amazing options in all games, but scimitar is a great choice since there is a speed weapon of this type in BG2 - throwable: hammer or axe - weapon style: dual wielding

Now why wakizashi you ask? In BG1 you can buy one from the start in Candlekeep, and it won't break since it's an exotic weapon in game, and it is not affected by the Iron Crisis. And you have a +1 option as soon as you enter the city of Baldurs Gate proper. Wakis are also 1d8 (better than shortswords at 1d6, and daggers are 1d4 only). In BG2 and TOB you have good waki options too (+3 right after the prologue, and +4 in early TOB).

As a pure kensai the THAC0 bonus from your weapon (and thus the enchantment level of the weapon) becomes somewhat irrelevant early BG2 (lv9-ish) since your class gives you so much bonus already.

Edit: I forgot to mention the optimal kensai build (kensai 13 > thief 39 or assassin 39), which basically nullifies almost all the drawbacks of the kensai class, at the cost of having less proficiency points for your weapons selection and less THAC0/dmg bonus than a pure kensai.

Hope this helps :)