As posted above, there are more theories on the "right" way to run a scraper spread than I know how to count. Part of this is due to different site conditions. Each technique has its place. Picking up scrapers with the scraper in 2nd gear is appropriate in good conditions, andwith consistent material, it is normal for the scraper to drag a few yards into the can on his own before the cat gets there, helps him slow down and makes things easy on all. Different methods depending on single, double, or even triple pushing. See ROP's post above. Yeager spreads are some of the most efficient around, using 2 and sometimes 3 D-9L or D-10N pushcats. 3 Cats shaves only a second or two from the load time, but the load is bigger, and not just from being piled on, but it is compacted into the can. 2 similar looking loads, one with 2 cats, one with 3, when weighed, will show about 2 to 3 CY more in the can with the extra cat. If they are making 120 loads per hour, the 3rd cat is worth an extra 240 to 360 cy per hour.
Al this goes out the window if you are in rock, or even worse, loose material with scattered boulders. Trying to run production like that in the rock will end up with cutting edge bolts sheared, floors peeled back, tire damage from the dozer when the scraper suddenly stops midway through your turn to catch it, and a host of other problems. In these conditions, I advise each scraper stopping and the dozer tagging up then load. I know its slow, but things stop completely when all the iron is broken. Sometimes, you can get away with making sure the scraper keeps the can up untill dozer is firmly tagged up, then dig in.