What does it do? Has anyone tested it?

yup;

http://www.uesp.net/wiki/Skyrim:Destruction_Dual_Casting

"What does it do?"
"yup"

Wait, what? Your link doesn't help either.

Lightning-based spells do more damage.

I got the chain-lightning spell yesterday, dual wield, awesome, and it has a long reach, too. I was able to hit dragons in flight (but not when they're way up, of course), killing enemy archers standing on walls of old forts. Just be careful if you have a follower, it will jump to them (that's stupid).

http://www.uesp.net/wiki/Skyrim:Magic_Overview#Dual-Casting 2.2 times the power for 2.8 times the cost. So it improves the damage the cloak does; 2.2 x 8 damage/second = 17,6 damage/second.

Wait, I thought my question was: "What does dual casting frost/fire/lightning cloak do?"

Edit: Are you sure it works that way though? Seeing as alteration buffs for example are increased in duration when dual cast.

Tested in-game, it still only lasts for 60 seconds when dual-cast.

Also interested in the question - another wiki (Elder Scrolls I think) says dual casting cloaks increases the range they affect, not the damage.

Dual-casting's effect depends on the spell school. For Conjuration and Alteration, dual-casting costs x2.8 magicka and provides x2.2 duration. For Restoration, Illusion, and Destruction, dual-casting costs x2.8 magicka and provides x2.2 magnitude.

The cloak spells are in destruction, so they should simply cause more damage.