I have been playing as a Kensai now for quite some time. I am intersted in dual classing him for the first time to see if the rumors of dualed Kensai Mages are true. At what level would it be best for me to go back to an older saved game to dual my Kensai. I have heard some people say at level 10 and others at 12.
I appreciate your advice.

If you have ToB installed (or experience cap removed) then dual at 13th level. This gives you the extra THAC0 and improves dual-wield attacks to 9/2. If not, 9th level would be fine. Anything after that is just extra.

Remember to scribe a ton of scrolls (potions of genius/mind focusing) and your levels will catch up quick.

Terrific! Thank you Mitro, I do have TOB installed and have an old saved game with my Kensai at level 12, so I can bring him up to 13 and dual from there. I also thank you for the tips, I wouldn't have thought of that. Mucho Gracias 8)

I dualed my kensai mage at level 15 I dont know how good it is yet because i havent gotten my mage level up high enough. im on about 14 right now. ill let you know in a few days, but its going pretty good so far