Rewatching the twitch video where we see the NIN apply Shadow Fang (and the Slashing debuff) they also use Duality + Armor Crush
I noticed that 3 numbers came up:
a 5K critical hit
a 3.4k normal hit
and a 1k normal hit
I am supposing the 1k one is an auto-attack, and the 5k and 3.4k are from Duality. This may mean that either they changed Duality so the first strike can Crit, or they removed the "Can\\\\'t Crit with Duality" part.
http://i.imgur.com/gzSc9wI.jpg
Snapshot at 1:45:44
Edit: NVM, not a guaranteed crit
http://i.imgur.com/HHFYQ1n.jpg
With Duality AE at 1:38:27
But it seems like they CAN crit
Edit 2: There is another one
http://i.imgur.com/qiHLRbP.jpg
Another Duality AE at 1:43:03
.SNIP