From the Compendium (emphasis mine):
Swarm: A swarm is composed of multiple creatures but functions as a single creature. A swarm can occupy the same space as another creature, and an enemy can enter a swarm’s space, which is difficult terrain. ...
The issue is the interpretation of the word "another".
If you think that it implies the singular, then a swarm can occupy the same space as one other creature. A second swarm would not be able to enter the square because it can't be in the same square as two or more other creatures, only one other creature.
On the other hand, English being the imprecise language that it is, "another" might be used more generally to mean "one or more". For example, if I were to say, "There isn't another person around for miles," I don't mean that there isn't exactly one other person in the area, I mean that there aren't one or more people around.
Ultimately this is going to have to be up to the DM (unless you're really desperate for a RAW ruling, in which case I suppose you could ask the English Language & Usage StackEx).