QUOTE(Maximum_Joe @ Oct 23 2011, 04:45)

Did some testing, having another equipment set with katana+wakizashi, and testing things around.
First off, I got more damage from the katana alone, thanks to my skill in 2-H proficiency, so when I added the wakizashi, my damage actually decreased considerably (but then the wakizashi wasn't the best one possible, but still).
I then switched back to my main set, but switched to DW and increased that proficiency a bit, which gave a damage increase to the niten ichiryu set. I then switched back to 2-H and increased that proficiency, again, it gave a small bonus to niten ichiryu.
So it seems to be some kind of composite of the two. But it seems that at least 2-H doesn not give half ot it's bonus, but much less.
I don't know if DW would then compensate for this (giving 25%-75%?), since my DW proficiency is pretty low, but at the very least, it appears that both proficiencies increase the damage.
For some numbers.
With this
Average Katana of Slaughter alone (and some mixed light armor) I got 1393,9 in base damage on my physical attack.
Then I add this
Fine Wakizashi of the Nimble, and my physical attack decreases to 1194,7 base damage.
The parry increase is also pretty small, especially compared to the value on that wakizashi.
Now I know that niten ichiryu doesn't give the parry bonus to the off-hand weapon that normal DW gets, but it's still a fairly decent number.
Parry increases from 17% to 21,2%, and since the katana has no parry value in itself, it means that a lot of the parry bonus given from 2-H proficiency is lost.