There's also Margaret Weiss/Tracy Hickman, if you're willing to count Dragonlance as epic fantasy. This pair is odd, as it has one clearly female name and one that is ambiguous. I had always assumed Hickman was female until I looked it up just now.

I don't know the readership breakdown on that series, but if you count the series as a whole, they have sold a trainload of books.

Marion Zimmer Bradley, too.