Roseby - Fury/Arms Warrior
Jobei - Elemental Shaman
Requiel's #1 Fan.
The sockets were added to the vendor so that people who used the socket in an lfr/normal weapon would be able to get another socket for when they got an upgrade. This had the side-effect of allowing dual-wield classes to use two at the same time, but there's been no developer comment as to whether that's intended as far as I'm aware. Personally I imagine it's not intended, but one extra socket isn't such a massive issue that they'll do anything to nerf it.
I don't know if I am so eager to upgrade a total of three one-handers. Since I have already own an upgraded Scimitar that's a lot of VP invested. On top of that the second Kil'rak would have to be better than the Scimitar plus whatever else I could have upgraded.
There's also the possibility that SMF might get nerfed relative to TG in 5.2 and we would have to upgrade a pair of Shin'kas too.
I probably won't invest in another Kil'rak until I know what 5.2 will bring especially since I'm already behind on upgrades.