Will i get X16 damage on both daggers if i use them both at the same time?

will this mean a backstab will do twice the damage it would with a single dagger?

yes and yes

as long as enemies don't detect you, you can backstab as many times as you want

sometimes i get a x 4 x 16 damage burst with dual wield daggers power attack.

awesome, i dont see any reason not to dual wield for a stealth character if thats the case

Pretty sure it's x15.

yes. I can add that I usually get the forced 1-shot deathblow animation though. (throat slitting with the dagger using the non-cutting side of blade, not using a double-edged dagger)

slap ye old db gloves on and double that backstab to x30 then run around shouting like a boss after you rock someone for around 1100+damage

Add 5/5 Armsman and an orc's Berserker Rage to that