All of the Additional Effect weapons have a base 10% chance of adding the effect. On +1 Versions of weapons, this increases to 20% (just like all of the other base status improve such as damage and delay). Purchasing a +1 version of an Additional Effect Katana (dual wield) raises the odds to 40% hit per melee round (two swings). After two melee rounds, the status effect will likely kick in.
On the "Weakens Defense" effect, the effect stacks. Many RNG rave about the effect of Acid Bolts have when stacked with Darksteel bolts and the Othinus Crossbow. The Yoto +1 works the same way. Not only do you deal more and more damage per hit that the effect kicks in on, but all other melee damage dealers also benifit from the effect. That makes these Katana very popular.
Remember that when you outgrow any equipment, you can always sell it back for what you purchased it for.
That's not how math works... if you have 20% chance the effect will go off, then you have an 80% chance that it will not... in each hand so to figure out the chance that either will proc you find out the chance that neither will proc and then subract that from 1 and then multiply for the percentage
so 1 - (.8 * .8) = .36 * 100 = 36% proc rate with 2 20% weapons
I can give you a little info upon your great post! (rated up) I just got 2 of these signed by my best friend who made them for free for me. I went from a weak hittin ninja (great tank) to a great DD ninja! What I did was use these with meat miths for more attack, then attacked for around 50's each hit in crawlers nest vs beetles and scorpions. I got defense down I'd say every 6th hit or so. Sometimes i got it back to back tho. Could easily see the difference.
i would pay good money(gil) to get 2 of these considring it weakens defense making it easier to fall an enemy. also since it's +1 you'll do extra dmg and hit faster.