A duo is a pair of things or people. Batman and Robin are a famous dynamic duo, but a duo is any two people doing something together, not just crime fighting. In fact, a duo usually entertains.
You can complain that you can't find a duo of matching socks, but it's more common to use duo for people who perform together. For example, Penn and Teller are a magician duo, and Laurel and Hardy were a comedy duo. If you and your sister play your ukuleles together in the subway for tips, you're also a duo. Originally, a duo was a song written for two voices, from the Latin meaning of duo, "two."