Doubts about some combat feats...

[Fenn Grunberg]

Hi, I'm a bit uncertain about the correct interpretation of a couple of combat feats:
1) Knife Mastery: the 2 additional dice of sneak attack damage are added only against flat footed, or surprised, etc. opponents, or against any opponent, whatever his/her/its conditions?
2) Flail Mastery: "If this attacks hits, it counts as 2 melee hits for this round": does it mean that I roll twice to determine the damage I score, or it's only useful to gain benefits from the Whirling Serpent Stance?
Many thanks,
Fenn

[Krensky]

1) Whenever you can apply Sneak Attack damage.

2) Pretty much it's for Whirling Serpent. A hypothetical Edge builder or some other option keyed or triggered off a melee hit would benefit from it though.