Cooperative Spell( Tome and Blood: A Guidebook to Wizards and Sorcerers, p. 39)
You can cast spells to greater effect in conjunction with the same spell cast by another.
Any other metamagic feat,
You and another spellcaster with the Cooperative Spell feat can simultaneously cast the same spell (at the same time in the round). You must be adjacent to one another when casting cooperatively. Add +2 to the save DC against cooperatively cast spells and +1 to caster level checks to beat the target's spell resistance (if any). Use the base DC and level check of the better 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 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. For example, four spellcasters (two wizards and two sorcerers) standing in a circle all possess Cooperative Spell. Three of them ready an action to cast fireball when the member with the lowest initiative takes her action, also casting fireball. The base DC of the Reflex save is equal to the highest individual save DC among the cooperative casters, as determined by their relevant ability scores or other feats (such as Spell Focus), special abilities, or items. In this case, one wizard has Intelligence 18, which ties with a sorcerer's Charisma 18, so the base DC is 17 (10+3 for the spell's level +4 for the ability score modifier). The final save DC of the cooperatively cast fireball is 17+2+1+1 or 21. Whoever has the highest caster level determines the base caster level check, which gains a +4 modifier.