Tonight i crawled around my old stuff on the PTR and found an interesting thing, the T9 2pc set bonus is the same as the T10...5% to all damage. I just found it as i tried on my Turalyon's Gaunlets of Conquest on the place of Fleshrending Gaunlets, and in the character window my base damage was higher with like a hundred and dps by 20.
So it seems if you equip 2xT10 and 2xT9 you get 2x5% boost...which is a lot.