From what I understand, usually one of two reasons:
1. To increase the size of a spawn. For instance, you have a prized male, and you want to make sure you maximize his spawns. 2 females each spawn should increase the average spawn size, allowing you more fry out of a given male.
2. You have a male that you want to pair with two different females for different reasons. This is my case. I've been wanting to spawn my opaques, but I've also been dying to x opaque and copper. 2 females in, 2 spawns, one go.
Oops, forgot to add. Those are the spawn reasons. Apparently it can also help keep everybody less nipped, because the male switches between the two. It also (supposedly) induces better bubblenest production.