No monster is "stackable" in the game. Including swarms.
The reason you'd want more than one swarm under your control at a time is because you're faced with a particularly large open area with a lot of foes spread out over more than a single ten-foot-square.
Does this carry over to Second Edition as well? Swarm trait in the 2e Beastiary RAW states that swarms may occupy the space of other creatures, implying that a GM could create what I call "spider cubes".
Sorry about this being in the 1e forum, but it's the only authoritative thread I could find for the subject.