Conjuration, casting the spell "Conjure Flame Atronach", perk: Apprentice Conjuration (half cost of this spell)

Perk: "Conjuration Dual Casting" = description "Dual casting a Conjuration spell overcharges the effects into a longer lasting version"

Without [Perk: "Conjuration Dual Casting"]:

• 1 hand casting - normal casting animation - 1 min duration - normal cost of spell (acording to description of spell)
  
• 2 hand casting - normal casting animation in both hand - 1 min duration - DOUBLE cost of spell

WITH [Perk: "Conjuration Dual Casting"]:

• 1 hand casting - normal casting animation - 1 min duration - normal cost of spell
  
• 2 hand casting - one big animation when casting (nice!) - 135 sec duration (so not x2) - more than double cost. It's either x2.83 or maybe x2 +50 (x2 +50 seems more likely)

I expected that 1 hand casting would be the same with and without the dual casting perk. Tests confirm this
2 hand casting without dual casting perk costs double, but doesn't seem to do anything

So has anyone else more information about this? I can't see that the Atronach is stronger when dual casting (with & without dual casting perk)

dual-casting only cause strom atronach to stay summoned longer than normal. It is same for all summoning spells. Furthermore, from i read from other posts, that they getting stronger on levelling scale. That is only assumption since i haven't tested it on my own regarding the levelling scale.

You can't dual cast without the perk. When you cast flame atronach in each hand separately, you are effectively casting the spell twice. Since you can't have more than one atronach out at a time without the associated perk, you are essentially immediately overwriting one flame atronach with the other when you cast from each hand at the same time.

Dual casting is inefficient and I believe that's working as intended. Unfortunately as it stands, it's inefficient to such an extent that there's very little reason to dual cast almost any spell outside of destruction with the impact perk, or maybe if you only cast a summon or buffs on yourself at the beginning and then swap to non-magic means for the rest of the fight.

Haven't tested, but if you would have the dual-summon perk, you would summon two atronachs at the same time if you dualcast without a perk.