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I'm starting BG as a fighter, and I intend to take him through ToB (I haven't played BGII at all before). I'm just curious what level is the best to dual with the least downsides. I've read that some wait until after the beginning of BGII around level 9, but the downside there is that you can't dual to a specialist mage (I'm not using Tutu or anything btw). Some say 7, but I'm not really certain why. Does anyone know a definitive sort of answer?
This question / problem has been solved by kmonsterimage
It doesn't matter whether you dual to specialist wizard in BG1 or not, when importing into BG2 you'll become an ordinary wizard instead, you may choose a kit for the fighter levels if you wish however. Be aware that berserkers can't be lawful and kensais can't be chaotic.

Most popular levels for dualing are 7 (just got the extra half attack per round and allows finishing dualing during BG1), 9 (most hitpoints, quite short dualing period at the start of BG2), and 13 (gained the second extra half attack and another +4 to hit but it will take very long until the dualing period will be over and even afterwards your character will still be 1,250,000 behind in XP (that's 3-4 mage levels) for the rest of the game).
I recommend lvl 9. 13 takes too long and 7 leaves your fighter a bit weak.
I'd recommend 9 if you're planning on using a full party, and 13 if you're using a small party. The XP issues from dualing at 13 are largely alleviated by having fewer party members, and by the end of the game you'll only end up missing out on a couple of level 9 spell slots, but you'll have a much lower THAC0, another proficiency, much better death and breath saves, and more attacks. 9 allows you to concentrate a bit more on being a wizard, though, since you'll switch over faster and get more spells by the end, so you might want to choose that one anyway if that appeals to you.
If your fighter levels as Kensai dualling at 9 becomes particularly appealing since you get another +1 to hit and damage, a little extra speed at level 8, and another Kai.