Yes.
Don't.
I tried that once when I'd just got my licence, nearly frightened myself silly. The a/c you're trying to photograph goes from a dot in the distance to filling your window to a dot in the distance again.
Please, please, get some formation training before trying this.
Of course it's possible that the instructor you mention is fully trained in formation flying techniques and you are the passenger doing the photography. But what about your friend in the other a/c? Has he had the appropriate training?
You might like to start making enquiries for a course at North Weald, if that's anywhere near you.
Cheers,
The Odd One