The reason for your inability to use Kensai abilities is explained in-game.
Until your character becomes a level higher in their new profession, the previous class you dueled from will be inactive. As such all bonuses and abilities of that class are deactivated until in such example:
Dualed Fighter lvl 6 to mage lvl 1 = inactive fighter class
lvl 7 mage + lvl 6 fighter = Fighter class reactivated, all abilities restored (except the slots of weapons available, that always remains the same after the first change)
I highly recommend against dualing above the halfway marker for the experience cap in
BG2.
This is not to do with mods, but just the way on how the dual-class system works.
I hope this clears things up a bit.
Classic Adventures - a Total Conversion compilation dedicated to bringing many of the old and new PnP modules into the wonderfully dangerous world that is Baldur's Gate II. Fancy link button to Downloads page.
CA Forum. Bugs to be squashed, feedback to be welcomed!
Website at: http://classicadventuresmod.com/
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When in doubt, lockpick a mudcrab!
Beware the nug conspiracy!