Let's take a look at destruction spell, exactly when you are going to use the FLAME spell.
Are you going to use it (single hand or both hand) in one shot, then wait half second or a second, another shot, then wait... or just keep the mouse pressed like a flamethrower?
Is it going to do the same amount of damage?

Up!

Usually on right hand only. I have on left hand either a summon, heal or ward spell (not a destruction spell). I only use it on both hands with a perk ofc when target is not directly attackin me (dragon attackin a guard.) Boosting my dmg a bit.

I don't know about dmg when you cast on both hands, and dont have a perk. But it does do more dmg to targets that are on fire.

I myself just keep pressin the button until target is dead.
:spotted owl:

This is what I was doing, keeping pressing, but a friend of mine told me it's much more efficient and makes more damage if you click and wait, click and wait.
Someone confirm ?

if you click and wait you get the initial 8 damage from the flame, then the damage over time ticks down, and so on.
While this is more damage per point of mana, its less damage per second of time than just holding down the button and getting 8 damage a second and the dot damage.

If you are running out of mana, and have a mage robe / invested in the half price perk, I would be surprised. Keep the button down for the extra damage.

If you are playing with flame spells in armor instead of a robe or dont want to invest into the novice destruction perk, tag them and let the dot do some of the work.