The difference between simple bend and unison bend is

that unison bend begins with playing two notes simultaneously,

one note is to be lifted as to regular bend performance

and other one is a stable note (no bending).

So when the bend note is lifted both notes begin having same

frequency (hearing unison effect).

Watch the video to easily get the idea

In order to perform this effect with full bend note,

stable note should be one tone higher than bend note.

For instance:

if you bend note at 7th fret on 3rd string stable note should be at 5th fret on 2nd string

And if you bend note at 8th fret on 2nd string the stable note will be at 5th fret on 1st string. It's because of different intervals between all these strings. I'd recommend you put the thumb of your left hand as I did on the next picture below it very helps easily play bend notes, and use at least two fingers to bend them, if you perform bend with your 3rd finger, involve also 2nd one. The difficulties are not necessary here!

As an exercise play follow pattern:

video example for Unison Bend Exercise