Not willing to try this out on my only computer, but it occurred to me that using multiple hard drive defragmenters might be a good idea, or might be disastrous. I'm not asking for which defragmenter is best, but if you could get better results from running different defragmenters in sequence.
Also, what if they were running at the same time? Deadlock? Livelock? Or maybe no problem, just finishing twice as fast (doubt this, as I/O is bottleneck I'm sure)?