I know it is possible to let the player control several summons at once using the CS, but i think I once read it is also possible to creature multiple summons using the same spell. If possible, how?
For example, I summon a frost atronach, then recast the same spell, without the atronach being replaced by the new one, instead I would end up with 2 frost atronachs.

only by means of a scripted spell effect which creates a summon that isn't tied to the spell effect. However, most summon scripts don't like leaving it open like that as it becomes rather hard to manage and easy to exploit to the point of causing permanent harm to your game. But yeah, it has to be scripted, and the scripting has to be designed in a way which the spell creates the creature, but other scripting eventually removed the creature.

In addition, you would need to change the Gameplay Setting iMaxPlayerSummonedCreatures. Without changing this, you will get a message saying that you have too many followers. It is also possible (without scripting) to summon multiple different creatures with a single spell, once iMaxPlayerSummonedCreatures has been changed. http://img256.imageshack.us/img256/6674/daedricmultisummon.jpg gives you an idea of what's possible: A Dremora Lord, a Dremora, a Spider Daedra, a Daedroth and a Clannfear. With this technique, you cannot summon two of the exact same creature (i.e. two Spider Daedras) but you can summon several different creatures.