In my experience, a Wraithknight works best when it is used as a "tank" to absorb lots of damage and to keep the rest of your army alive.
The main strength of a WK (and about the only one it has now) lies in its capability to combine shooting AND assaults to great effect: once it has closed with the main enemy position, it can shoot something, then immediately assault something, then next turn step out of melee, shoot again, then assault again. Only fighting that way it can actually cause enough damage to justify its huge point cost, and can pose some serious danger to the enemy.
So, when you use it to spearhead your attack and move it in decisively, it becomes a good "tank" -
because it poses so much threat that it cannot be ignored. But, in order to be efficient as a "tank" it has to be able to absorb a lot of damage, and for that
1) it needs Fortune AND,
2) preferably, it needs to stay undamaged until it is Fortuned.
This makes a Skathach a very perspective choice on paper, because it can stay off table till your turn 1, and can IIRC even attempt an assault right on turn 1. As for two Skathaches - I personally would never do that, but that's because I like my armies to consist of many diverse units, and tying so many points in just two identical models is way too boring for me
But from competitiveness standpoint it might be good.
The only thing that makes me doubt the efficiency of Skathaches is that they really need two guns to be good in shooting, and having two guns they have no invulnerable save, which is dangerous. But they are still worth trying imho.