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).
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