This has been bothering me for a while, but if I have dead thrall and use both triggers (LT and RT) does the level cap increase for raising thralls?

Do you mean if you were to, say, make conjuration legendary, if so, you still keep the skill and conjuration increases in combat only if you raise a thrall.

Edit: also if you dual cast, the amount of exp gained is more than if casting it one handed twice, but mana is depleated more, so i go single casting for leveling.

I mean't named NPCs like Ulfric only level up so much, before they stop leveling with the player and cant be raised. What I want to to know, is if I use dual cast and have necromage, will the level cap to raise a npc be increased?

Oh, no it wont, but there is J'zargo of the College of Winterhold, he levels up with the player with no cap, or atleast to level 81 (dont know if legendary affects it since the level cap is essentially removed) and the two brotherhood initiates also level with the player.

If that help, but those are just the ones i know there could be more.