Does anyone have some numbers for the dual casting of spells?
i.e. flames does 8 dmg/sec, and one flames spell in each hand gives off 2x8 dmg/sec, but as most of us know by now, when you press both MB1+MB2 at the same time you get in this example one flame out of both hands combined, which is supposed to be stronger.
But what I really would like to know is how much stronger the combined spell is compared to two (weaker) spells..

Checked the wikis one more time, and it's apparently called overcharge..
Still no one who has an answer?

Might it just be doubly effective? And doubly cooler?

If you don't have the dual casting perk for the school of magic for whatever spell you're using, then it's just twice the effectiveness, since you're casting them side-by-side rather than combining them http://myona.com/wp-content/gallery/skyrim-new/skyrim_1010_01.jpg. If you have the perk, it's more than twice the effectiveness of the standard spell, but by how much I don't know.

the perk that allows dual destruction casting is devastating...

Yeah, I have the perk (the first in the destruction tree i wanted) and dual casting (overcharging) fireball at the low cost of 42 magicka is just overpowerdly awsome! I know it does some amount higher dmg than sending off two fireballs at the same time, but it would be nice if someone could do some research.. Feel free to post links folks!!

I tried testing it a while ago. It seemed like the spell did 225% effect (16 point heal was a 36 point heal when overcharged). I only really tried Restoration; I don't know if other magic types are the same.

Nice find!
I'll try it myself next time I get some time to play
Is there any other spells that can be used to check this accurately?

Well in terms of destruction, it is helpful when you get the impact perk as you can keep enemies at bay almost indefinitely while dual casting.