Type: Metamagic
Sources: Complete Arcane
Forgotten Realms Campaign Setting
Player's Guide to Faerûn
Tome and Blood
You can simultaneously cast a single spell twice.
Prerequisite: Any metamagic feat.
Benefit: Casting a twinned spell causes the spell to take effect twice in the same area or on the same target simultaneously. Any variable characteristics (including attack rolls) or decisions you would make about the spell (including target and area), are applied to both spells, with affected creatures receiving all the effects of each spell individually (including getting two saving throws if applicable).
A spell whose effects wouldn't stack if it was cast twice under normal circumstances will create redundant effects if successfully twinned. For example, a twinned charm person doesn't create a more potent or long-lasting effect, but any ally of the target would have to succeed on two dispel attempts in order to free the target from the charm. As with other metamagic feats, twinning a spell does not affect its vulnerability to counterspelling, so a single successful counterspell negates both instances of a twinned spell.
A twinned spell uses up a spell slot four levels higher than the spell's actual level.