When running single engine scrapers, you're completely dependent on your dozer for loading. If it goes down, the whole operation stops. While tandem engine scrapers can keep running if one goes down.
There's strong justification for single engines with a dozer pushing being more fuel efficient, but that really hinges on the models of both scraper and dozer (newer vs. older).
On slopes from a 3:1 or steeper, tandem engines typically beat out the singles.
Advice from a scraper foreman: If you're putting the scrapers through shot rock, most of the time a single engine with a big dozer is best. For 621's, it'd be hard to beat a D9, if not a D10. If you're running 627's, run them single through the cut and push with a dozer.
From my experience, running the tandem's push/pull through shot rock results in a lot of flat tires. If you run them single, have the operator keep power on the tires, but let the dozer do the work. It'll save a lot of scraper tires in the long run.