A kensai/druid is a perfectly legitemate dual class as long as you start with the kensai. He/she will need 15 in the prime requisit (i.e. strength for the kensai) and scores of 17 in the secondary class (so for a druid, WIS and CHA need to be set to 17). The alignment of the kensai must be true neutral. Then he can dual to a druid. You cannot dual into a kit, only a pure class (unless you want to cheat and use shadow keeper)
I think a Kensai/Druid would be quite cool. Place slots into scimitar to maximise those attacks per round (think Belm). Staff of the woodlands is a good idea and will vastly improve the kensai's AC. Don't worry about sacrificing defence since kensai's cannot wield shields (even when dualled), concentrate on dual wielding to get as many attacks as you can from the kensai since each attack is going to hit hard!. Dual-class to druid at level 13 IMO to get the extra 1/2 attack per round and give yourself decent melee skills.
I'm soloing a single class Avenger Druid at the mo and it was a total nightmare earning 1,500,000 XP to get from level 14 to 15 and the game was very tough since druids lack melee skills. Things are beter now I can summon Elemental Princes and Devas to fight side by side
