Just wanted to share my build, not at home to paste the exact training, but the essentials are Edged, TWC, Ambush, Stalking/Hiding, and I believe 30 ranks Multi Opponent Combat. CM too, because the skill you are after is Whirling Dervish wanting it to trigger 80% of the time.

I change up my weapons when I get bored, right now it's that 6x golvern wakizashi with unbalance flares which is amazing, and a high enchant dagger. But it usually doesn't matter what combination I use, things just die. So 80% of the time if I death crit the first creature, my offhand weapon aims for the same spot and crits a 2nd creature in the room. Toss in there an occasional mstrike, and it's pretty fun. I'm probably missing something. Mainly just wanted to share another option with you.

You don't need the weighting.
Flares are nice as a backup.
If you just like the look of katanas I would seek out an old falchion-based one.