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
Diego Torres
Architect-CG Artist
Architectural Concepts and Visualizations
ArchiCAD 4.55 - 24 http://www.sightline3d.com
one more thing which is often forgotten, it really helps to pinpoint the exact location of where the photo was taken including the RL. If you then place the ArchiCAD Camera in this exact location you are starting of on the right foot and can tweak from there. As all ready mentioned you really would need to know the focal length used in the photo. saying this, it is not a strong point of ArchiCAD and most professionals use additional software / plug ins. Least but not last I have yet to come across a Photomontage which was not adjusted in post production. It's necessary as there is always "stuff 'that needs to be removed, hidden away, added and adjusted.