You sure can. As soon as you dual, all experience after the last level is dropped. So if one of your Kensai is level 9 and has 400,000 exp., as soon as you dual-class, all exp. after 250,000 (last level up) is dropped.
Personally, I would have the Kensai you planned to dual to a thief dual at level 12; that is, unless you don't have any thief.
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"This place will be your tomb."