I have an idea for you. I use a similar method in C4D:
• PhotoRendering Settings. Add the final existing picture you are going to use for the photomontage. Try to use a size that fits your screen.
• 3D Window Settings. Background as in PhotoRendering.
• Move, rotate, pan camera until you get a fair result.
I believe this might help a little more than using two screens.
Cheers!
Diego Torres