You know I thought also that all of the summons were connected also, and that releasing it would release Madara's summon to. But I've been thinking about it, there are two reasons why this isn't true I think. First, remember when he summoned the sixth coffin in front of Tobi, the Madara coffin. When Tobi was walking away he unsummoned that coffin, but the other coffins with the Akatsuki members were still standing there. The other part, is when he unsummoned Deidara when they attacked the turtle island. Yes we don't know for sure if he just unsummoned Deidara, but it would stand to reason he just unsummoned him.