D&Dtools1

Benefit

A spell altered by this feat takes effect twice, as if you were simultaneously casting the same spell two times on the same location or against the same target. Any variables in the spell (such as targets, shaping an area, and so on) apply to both of the resulting spells. The target suffers all the effects of both spells individually and receives a saving throw for each. In some cases, such as a twinned charm person, failure of both of the target's saving throws results in redundant effects, although any ally of the target would have to succeed on two dispel checks to remove the effect. As with other metamagic feats, twinning a spell does not affect its vulnerability to counterspelling. (For example, countering with an untwinned spell doesn't negate just half of the twinned spell.) A twinned spell uses up a spell slot four levels higher than the spell's actual level.