From testing with one of my poor huscarls in the basemant of a house I think flames is by far the best bang for your buck if you can survive the extra time it takes to kill something. Specifically for when you have more than one enemy attacking you at a time. Something I learned, ironically from Torchlight, was to drag any melee attacking you to the nearest ranged/caster and using it as a cone AOE. Flames hits everything in front of you in a cone. Another thing to note, and the reason I tested in the first place, is casting the spell singly from both hands does WAY more damage than dual casting with the perk. Part of the trick seems to be not holding the buttons down but casting left right left right etc. It has a slightly larger burst of damage when it's first cast so it is better to not just hold the buttons down. Dual casting doesn't do as much damage per mana.
(It took me a full bar of mana to drop Lydia when dual casting, not even half when using the above method)
I had a bit different of an experience. When dual casting, it charges your spell into doing an extra 25% damage, at the expense of extra 20% mana cost. So if that is true, dual casting will give you an extra 5% damage boost. Very little but its still there.

the best way that I have noticed to do damage with destruction magic, is to use bursts. For example, I will use the flame spell. The basic flame spell. I never hold it down as a continuous stream. I do small bursts for greater mana conservation. Just burst it for about 2 seconds then release, after you release their health will still go down for a second or two. Then burst again. I keep up this same process. If you don't dual cast, its the same thing, just blast them with a second or two of heat, wait until their health bars stops going down from the residual damage, then blast again. I think I'm saying pretty much the same thing as you did but with using dual casting. If dual casting does what its supposed to by adding the damage of both spells into one, for example 8 fire damage spell dual casted should be a 16 damage spell + 25% damage boost from dual casting (in this case that's an extra 4 damage, 16 divided by 4 or 25% is 4). So if everything is correct with the programming, then dual casting flames would result in a 20 damage per second spell. I'm fairly certain it turns out that way, but hey I could be wrong.

and don't take offense that I showed the numbers in the equation, I wasn't trying to imply you needed the numbers. I just added them because some of the newer players may not understand how the dual casting works and numbers was the easiest way to show.

But other than that I think we both have the right idea of bursting with the spells. The only difference is I use dual cast. But like I said, I could be very wrong. Also, nice point on telling him to line up targets for the cone effect. That's a good tip for conserving mana and killing faster.