Oeh!
And before i forget .. there's another very good reason to dual your kensai at lvl 13 ....
Lvl is the max. dual lvl, at which you mage will still be able to get all spell-slots.
IIRC, you can get your kensai/mage upto 13/28
At lvl 28, a mage gets his last couple a Xtra spell-slots, you will not get any extra through lvl 29-31 ... so lvl 13 is a briljant lvl to dual at ... Max. figter-power and Max. amount of spells