Twin Spell

 
Prerequisite(s): Any Metamagic Feat (See main index)
 
Benefit: When you cast a twinned spell, it causes the spell you cast to take effect twice, simultaneously. If it is a damaging spell, roll the damage twice, if it has an effect that stacks, it will stack. Spells like charm are not doubly strong but would require 2 saves to break. A twinned spell uses a spell slot 4 levels higher.
 
Source: Complete Arcane