I've been playing a dual wielding character and i added the decap perk; and i've killed dozens of NPCs already but not one of them got decapped :sadvaultboy: does it work for dual wielding power attacks? Or does it have to be single weapon power attack?

You can indeed.

Dual wielding has the most awesome decap of all so what you do is standing not moving power attacks too get it. (Thougn sometimes it works anyways without power attacking.)

This.

But, it procs around 1000 times more on 2h and 50 times more on one handed only attacks.

My dual wield orc is shelfed now.

But I had the perk for a good 20 hours. And during this time I explored and fought alot.
I ONLY used dual wield power attacks. And i got the decal ONCE.

With 2h, it procs every now and then.
I think 2h although slow and sluggish, have an insane higher chanse to crit.
There are also no mobs that I can get one shotted by even on adept as 2h mobs.
And i have been decapitated by 2h npcs so many times I lost count, while seen it maybe once for npcs using one hand. Actually, I dont even think once.

I suppose you mean the normal power attack? because the character moves slightly forward during the normal power attack.