I suspect the intended complexity is of the "Know the fight" sort. Each summon has a different role based on what you need at the time. Looking at the Live Letter demonstration, from activation of Demi Bahamut, there is about a one minute cooldown. Each of the summons has a 30 second duration. Minus the 15 seconds of Demi, you're only using one and a half summons if you want to get Phoenix up as soon as possible. One of the summons isn't getting used, and the other is only getting partial use.