^
Perhaps if the egi could mingle their elements together in order to create an all new effect while simultaneously allowing the SMN to summon another Egi that corresponds with said particular combinations of elements?

Example: Summon Garuda. Causes a tornado that consistently does damage for a certain period time. Then next summon Ifrit. Resets Garuda's tornado while also boosting its damage and transforming it into an inferno twister. Now you've unlocked the ability to summon their Eden fusion that causes massive damage upon its summon, so long as you do so within the remaining time limit Garuda and Ifrit's inferno twister combo lasts.