Play two notes simultaneously and bend the lower sounding note up to the desired pitch. Some unison bends also bend both strings.
Listen to a unison bend
The image below shows how one note is bent up (3rd string, 3rd finger) while the other in held into position (2nd string, 4th finger).
Unison Bend Riff 1
This riff is bending the note (marked in red) on the 3rd string 7th fret up 1 tone (this is a full amount) and the note on the 2nd string 7th fret up a semitone (this is the value of 1 fret). The notes on the 5th fret (marked in blue) are bending a quarter of a step (this is half a semitone). Do this by using your 1 finger and pull your finger down, just enough to put the notes a little out of tune.
Listen to the riff example