If you dual at 12 you get the extra 1 popint of thac0/dmg of the Kensai and another prof. point; if you want the extra half attack you have to wait till lev. 13.
At what lev. to dual depend on how is big your party and if you have or not ToB:
if you are playing only SoA with a six party then is better to dual at 9, if you have ToB and/or are using a small party (max 3-4 char.) then lev 13 is the best choice.
Regarding the comparison with the berserker-mage (and a lot more about the kensai-mage) do a search and you will find some very good threads of Littiz (the specialist of this class).