No, until you get a higher level as mage that the one you have in kensai you cannot use any of the kensai skills (use of a katana as a example), after getting a level higher in mage that the kensai you will get those back.
Once you have obtained one level higher in the mage class then what you dual-classed at, you will get your abilities back.
Example you dual class your kensai at level 10 to a mage, once you reach level 11 in the mage, your kensai abilities will be reactivated.
Just Dual classed my Kensai to Mage and i cant use my Katana no more? 
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