the one which makes spells stronger when you do em with both hands. does it also affect the damage of the master spells which are 2-handed anyway? otherwise id leave that perk out. thx.

I don't think so, but you shouldn't leave the Dual-Casting perk out if you plan to use Destruction later on.
It leads to the Impact perk, which let's you stagger almost every enemy in the game, so if you plan dual-wield spells anyway, that's just too useful to ignore.