I love the screenshot feature, but unfortunately it doesn't pair up well with trainers. What I did was re-assign the screenshot buttons to hold LB, then press RB to take a pic.
This way you still have normal controller functionality (I can't think of any games where I'd need to press both buttons at once), you can activate the trainer, and still take snazzy pictures =P
If that doesn't work... maybe get another trainer from someone else that doens't use back+start?