I've just recently started playing and I am looking for advice on whether or not I should dual class my character. Right now my character is a 10th level Kensai and I'm working on the Windspear Hills quest. Based on my stats, the only option for dual classing is a thief. One reason this seems appealing is that I would reach 11th level as a thief (and regain my kensai abilities) earlier than I would reach 11th level as a fighter (220k xp as opposed to 250k xp).
I also have a question about the rules. If I choose a proficiency in my new class that I already have in my old class, are my proficiency levels added togeter when I regain the abilities of my old class?

If this is your first time playing BG2, I would recommend you to export the current character and continue playing as Kensai in the current game. Later on if you want to start a brand new game, you can use the import function to import your lvl 10 Kensai and maybe dual him at lvl 10 or lvl 12. But if this is not your first ever game, then why not try dualing?

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Long live H22A1!!!!!

I think that if I were to play a second time I would distribute my initial stats differently so that I could choose mage or sorcerer as a dual class with my kensai. Right now my intelligence isn't high enough.

Dual clas you´re kensai into a mage and you get the strongest fighter in the game, if this is you´re first time however i woudn´t recomend it.

I see, you can also start a multi-player game and use two PCs. That way, you get both your original Kensai (at lvl 10, if you exported him at that level) and also a newly created character.

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Long live H22A1!!!!!

What about proficiencies? For example, right now I have 3 slots in long sword, 2 slots in two weapon style, and one slot in hammer (hoping to eventually find Crom Freyer). If I switch to thief and choose, as my initial proficiencies, one slot in long sword and one in two weapon style, what will happen when I reach 11th level as a thief? Will I then have 4 slots in long sword?

To answer your question ...NO.
If you have a fighter who is proficient in daggers ,and then dual to mage and choose a proficiency in daggers,when you get your fighter skills and proficiencys back you will still be only proficient in daggers and would have wasted a proficiency point. ...Understand?

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"The temple is I and I am the temple".