Dual casting spells

When I dual cast a spell (casting with same spell in both hands):

• How much magika will be consumed/needed to cast the spell (just the double of a normal one?)

• How is the spell affected? (How much damage does it deal? Or how much does its heal?)

• How are runes affected by dual casting? ("Trap"-Runes on the ground)

Of course given that you got the perk that allows dual casting in the school of magic you want to cast the spell.

How much magika will be consumed/needed to cast the spell (just the double of a normal one?)

It seems that dual casting cost a LOT more than just double the normal cost.

Single cast fireball - 42 magika
Dual cast fireball - 117 magika

Single cast chain lightning - 49 magika
Dual cast chain lightning - 137 magika

Single cast fire rune - 74 magika
Dual cast fire rune - 206 magika

Single cast conjure familiar - 44 magika
Dual cast conjure familiar - 122 magika

In all, looks like you will be using roughly 2.8x the magika cost of the single cast spell.

How is the spell affected? (How much damage does it deal? Or how much does its heal?)

For conjuration spells, dual casting doubles the conjuration life span of the spell.

For destruction spells, here are some damage values:

I used Cicero as my dummy target since its always fun beating up a clown.

He started out at 375 health (you can see a NPC's health by selecting them via the console and typing in getav health). You can also use this to see every other skill/stat of theirs. I avoided fire spells since they can burn and throw off some numbers with its burning damage.

Single ice spike: 25 damage (exactly matches the expected value I see by selecting the spell)
Dual ice spike: 55 damage

Single lightning bolt: 25 damage
Dual lightning bolt: 58.23 damage

Overall, it looks like damage is increased by roughly 220%

Keep in mind that impact will knockback the target if you get the perk.

For any other duration spells, the duration is doubled as well but the effects remain the same (stoneflesh still boosts armor rating by 60).

Dual casting the ward spells doubles the armor bonus.

How are runes affected by dual casting? ("Trap"-Runes on the ground)

1 handed Frost rune damage: 50 (matches expected value)
1 handed Frost rune attempt #2 35 damage
1 handed Frost rune attempt #3 22 damage (lol)

2 handed Frost rune damage: 64
2 handed frost rune attempt #2 55
2 handed frost rune attempt #3 75

The damage is not quite doubled, but the damage itself from the rune varies greatly due to the location of the rune and how close the target is to the center. Its entirely possible the rune damage is also doubled, but its very rarely that you actually hit for the max amount.

In terms of the AOE on spells such as Fireball, it seems like the surface area of the blast is, indeed, doubled. It just doesn't look like it due to geometry, but the AoE of a 15m diameter blast would be doubled with something like a 21m blast. Surface area increases with the square of the radius, so 1.4 times the radius doubles the surface area. That said, dual casting fireball feels like overkill for me,but the knowckback effect of the Firebolt, Ice spear, etc. is rather effective.

I've played around with this a bit, and it does appear that spells do more damage when duel-cast rather than 2x single cast, but not by a lot. It's hard to judge since I don't have numbers on the enemy health though.

I saved my game so I can re-load and try over on the same enemy to use as a control, and while it died in a single duel-cast ice spike, it had maybe 10% hp left when I just hit it with 2 single cast ice spikes. Again, it's really hard to judge with no numbers, but it does appear to boost damage, at least by a little.

As for magika use, it seems to cost roughly 3x the amount of magika compared to a single cast. It seemed to fluctuate for me between 2.7 and 3.1 depending on the spell. You can see this as well with the other answer above listing the magika cost (fireball for example, 117/42 = 2.8).

So it looks like it basically does 2-and-a-bit times as much damage, but costs 3 times as much magika. This would result in a slight boost in damage per second, but a drop in damage per magika. Combined with the impact feat, it seems to be useful situationally.

I haven't tested healing at all, so I can't comment on that.

Duration spells are simply doubled. That one was fairly easy to test. Armour spells and such don't appear to boost your armour any more than a single cast would.

yx's conclusions are accurate. According to modders who have examined (and altered...) the relevant game settings, dual casting in vanilla/unmodded Skyrim multiplies magicka cost by 2.8, spell effect by 2.2, and casting time by 1.0.

It depends largely on the traits you get from leveling up: dual cast on summoning make them last longer, destruction increases the damage or effect, or both, such as a spell that deals fire damage and leaves the target burning. It may damage more and leave the target burning longer... stuff like that.