Me and my hubby did the diet for a short while together although he now does Slim Fast. I found it good when we were both motivated but if one of us had a wobble we generally both had a blip together lol.
He did lose weight LOADS faster than me though which was annoying!
Yeah we did Cambridge together last year but gave up after my wedding and went crazy with food! Gained 2 of the 3 stone I lost so am back to do it again.
I do it with my mum, have found exactly the same problem, if one of us has a blip the other uses it as an excuse to come off too. We are bad influences on each other!