Dual-Casting
Within each school of magic is a perk granting proficiency in 'dual-casting' spells. Without this perk, casting a spell with both hands has the same effectiveness as though cast normally, but twice. However, once a dual-casting perk has been taken, relevant spells can be 'overcharged' via dual-casting, resulting in a single, more powerful effect. The formula seems to be: 2.2x spell effectiveness for 2.8x spell cost (remember that we're casting two spells at the same time so 2 is the base value here, not 1).
Whether dual-casting is worth it or not depends on school. For example, without the dual-casting perk, equipping two Frenzy spells in both hands is completely pointless, but equipping two Firebolts will double the damage output. Generally speaking, the school which benefits most from dual-casting is Illusion, as dual-casting increases the maximum possible level of affected creatures and the effect stacks with perks like Kindred Mage or Animage. The Alteration and Conjuration schools, and protective spells from the Restoration school, gain duration only, which is of limited use. Healing spells actually lose effectiveness when dual-cast, as they heal only 10% faster but for 40% more. Similarly, Destruction spells are more efficient when cast separately, unless you manage to push the magicka cost to zero, leaving you with 10% additional damage at no additional cost. The Destruction school's "Impact" perk causes dual casting to stagger an opponent, and may be well worth using regardless of increased magicka cost.
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