*study* this table carefully first before making any decision:
http://www.geocities.com/sixospades/bg/exp.htm
usually people dual Kenasi to mage at either lvl 9 or lvl 12. If you have a full 6 people party (=leveling up slower), probably at lvl 9 since you can get the kenasi ability back faster. if you have fewer people in the party, lvl 12 is good in that you can get 5 stars for your Kenasi for one type of weapon and 3 stars for one weapon style (good combinations are katana + two weapon style and staff + two-handed weapon style). However, look at the table, it'd be very late in the game when you get the Kenasi ability back if you play default SoA (no ToB and mods to give opportunity to get more exp).