How do I duel class through shadow keeper?

there may be a quicker way, but why don't you use sk to set the level of your first class, load the game, dual class using the game interface, save exit & sk.

Koz-ivan, I think that's also the easiest way, except when you're trying to create a dual-class that is prohibited in the game, such as a Ranger-Thief or a gnome Necromancer. I'm not sure if dodgy kits like that will crash your game, though.

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Melusine, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
http://www.angelfire.com/anime2/memnoch/mel1.gif
Your voice is ambrosia

Now..if im right ,in two more i'll be Elminster.Oooeer!

I think Koz-ivan has the best idea,and remember you still need to be human.