Cooperative Spell( Complete Arcane, p. 76)
You can cast spells to greater effect in conjunction with the same spell cast by another individual.
Any metamagic feat,
While the two of you are adjacent, you and another spellcaster with the Cooperative Spell feat can simultaneously cast the same spell at the same time in the round. Add +2 to the save DC of cooperatively cast spells and +1 to caster level checks to beat the target's spell resistance (if any), using the higher base DC and level check of either caster. A cooperative spell uses up a spell slot of the same level as the spell's actual level.
For each additional caster with this feat casting the same cooperative spell simultaneously, the spell's save DC and the bonus on the caster level check both increase by 1. When more than two spellcasters cooperatively cast a spell, each must be adjacent to at least two other casters involved in the casting. For example, two wizards and two sorcerers standing in a circle all have Cooperative Spell. The fi rst three in the initiative order ready an action to cast fireball, casting the spell when the fourth does. The base DC of the spell's save is equal to the highest save DC among the cooperative casters (as determined by relevant ability scores, other feats, special abilities, or items) +4 (+2 for the first cooperative caster and +1 for each of the other two). As well, whoever has the highest caster level determines the base caster level check, which gains a +3 bonus (+1 for each cooperative caster).