Before I took the dual casting perk in the destruction skill I would actually cast two spells when it was loaded in both hands. Now that I've taken the perk it only shoot one projectile and so far I'm not noticing a damage difference. Is it bugged, or is it just my game?

Note that the dual casting perk is magicka inefficienct as it means only 20% damage increase for 80% higher magicka cost. Unless you have a lot of magicka or a lot of casting cost reducing gear, it is more efficient to cast two normal spells with both hands. However, dual-casting has two advantages that, situation depending, may outweigh the additional cost: the DPS of dual-casting is higher, and the Impact perk provides a guaranteed stagger dependent on the spell. The stagger effect has no effect if the target is already staggering, thus a good balance between efficient damage and shutdown the target is to alternate a dual-cast with two quick double casts. Impact does not work with any constant 'stream' spell: the Novice spells or the Wall-type Expert spells. Master spells do not cause Impact stagger.

It doesn't do much more damage. It's the impact perk that will make the difference, it staggers everyone.

As the perk says, it's an "override," or basically turns what would be two projectiles into one stronger projectile. Honestly, the boost in damage is negligible (I think it does 2.2x damage, where normal dual-casting is 2x damage obviously) but it greatly increases the mana cost (I think 2.8x). If you're trying to maximize output, it's a perk you want, but it doesn't effect master-level skills (which use both hands by default) -- although the master spells really svck, so...

Really, magic is just a freaking mess. But there's the figures.