This might work:
Start out as a Kensai, and dual to thief when you reach a level you're happy with.
Then use SK to add in the assassin abilities level by level.
This way, you'll have all the kensai's armour restrictions (until you get UAI) together with his THAC0 and damage improvements, but you'll benefit from the extra thieving skill points when you level up.
+ edit +
Never mind - I just tested this, but I was stuck with 5x backstab.
I guess you're not meant to have multiple kits for a reason
+ 2nd edit +
I just looked at the other thread.
Maybe you could copy and paste the affects info for the backstab multiplier of an existing high level assassin?
Combine this with the bonus abilities from my original suggestion, and you'll have yourself a mega-overpowered backstabbing rapemonster of the highest calibre
