It's awfully common in fantasy these days to write a series of three or more books. I started trying to think of two-book series, and actually came up with a few. If you want to read more than a single-volume work, but don't want a huge investment of your time, here are a couple:
Flesh and Spirit and Breath and Bone by Carol Berg
Sailing to Sarantium and Lord of Emperors by Guy Gavriel Kay
Dragon Keeper and Dragon Haven by Robin Hobb
A Man Rides Through and The Mirror of Her Dreams by Stephen R. Donaldson
Writing a duology (or duet, you see both terms) is rare. Are there any I'm missing?