Source: Races of the Dragon
You can use two lower-level spell slots to cast a spell one level higher.
Prerequisite: Ability to spontaneously cast spells.
Benefit: You can use two spell slots of the same level to cast a spell you know that is one level higher. For example, a sorcerer with this feat can expend two 2nd-level spell slots to cast any 3rd-level spell he knows.