Prestige Class: Archdevil Ascension V

Your Archdevil Fiefdom benefits you additional fiendish cooperation. You may use concentration to maintain two creature summoning spells simultaneously. 

EX: On turn 1 you cast a creature summoning spell (Infernal Calling, Summon Elemental, Planar Ally, etc.), on turn 2 you cast another creature summoning spell without ending your concentration on the first spell. If you would lose concentration by taking damage you drop both spells. You may choose to end one creature summoning willingly while maintaining another.