A client wanted to document the condition of an empty factory building. I spent a day, with Joe, my assistant, walking though 2 levels of a city block size building and on its roof with a Canon 5D MkII and 2 Canon flashes. A Canon 580exII Speedlite was mounted on the camera, and Joe took a Canon 550ex Speedlite mounted on a Manfroto monopod. The second light was set up as a slave, and was held about 20 feet away from the camera to light up the vast expanse of the building. Both set at manual. Together they provided good coverage, but sometimes a third light would have been helpful in really large rooms.
Getting focus in a dark room with drab walls was sometimes a trick. We had brought a small flashlight, and shining that on the far wall provided a well enough lit target for the camera to focus on. We used a 24-70 4.0 L lens inside and a 70-200 2.8 L lens on the roof. We shot about 800 images in an 8 hour period. There is an interesting waterfall and river bank adjacent to the property, and for that we also shot several video clips with the 5D with both lenses.