Image processing with multiple shots of photos

Having two buildings, A and B, distant to each other of 20 km. I want to check the existance of any object in a volume in the space between the two buildings. A teacher in image processing gave me a solution using two cameras (But I still not understanding that method). 1. Put one camera above each building that could see each other 2. Make a multiple shots of photos from both cameras. 3. Via a specific processing of all images, I will be able to detect any object in the volume V between the two buildings. Could anyone explain to me how to use this mothod to detect all objects within that limited space between A and B? Thanks so much for any help