(Perhaps OpalCat should be answering this.)
How many songs are needed for something to be called a medley? I say three. I’ve heard of two songs being combined, but it doesn’t quite seem like a medley. Not sure what it should be called, though.
Two songs sung right after each other are often called a medley. It’s whatever the performer decides to call a medley.
If that bothers you, you could just say the two songe segue into each other.