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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)?
No, it is a bad idea. Different defragmenters have slightly different ideas about where to move certain bits of data to. This means that if you defrag using product X, and then using product Y, you'll be undoing some of product X's work. If you were to then run product X again, you'd go back to product X's version of defragged and product Y would consider it not fully defragged.
Running them together would be even worse as they'd be fighting each other (if it worked at all due to locking).
The best thing to do is stick to a single product.
Don't run them in the same time! They might implement different relocation algorithms. This might lead to one breaking what the other has fixed. Also, it will grind your computer to a halt.
Just use one disk defragmenter if you need to. Find a good one that does its job (the one built-into the OS is good enough) and stick to it. If it says the disk is no longer fragmented, there's no point in second guessing it.
No, as others said, you shouldn't try to run different defragmenters on the same data, and some can be unreliable. I tend to stick with the filesystem tools that come with the filesystem itself these days (that is, usually the tools supplied with the OS), as filesystems are quite complex lately, and updated regularly in various ways.
Also note that modern filesystems automatically manage fragmentation to some extent or another, and defragmentation is starting to become less necessary. The official Microsoft training materials for XP actually say something very similar to "Defragmentation isn't really necessary with NTFS, but it does make users feel better, as it's something they can do themselves to improve their system."
I highly suggest not to run 2 defragmenters at the same time. It will definitely not save time, as the limiting factor is the hardware. They will get in each others way.
The defragger that comes with Windows is not exactly clever when it comes to ordering the files. It consolidates each file into one block but does not move the files which are regularly accessed together. That's why sometimes, an alternative (like the free-of-charge MyDefrag or the free Jkdefrag) comes in handy.
OTOH, I don't defrag unless
the system is completely set up (OS, drivers, SPs, apps & data),
31 days later when the unused data count as "old",
and from then on about 2 to 4 times a year or 31 days after another SP (they do tend to slow everything down a lot).
Even weekly defrag schedules are, unless you are moving LOTS and LOTS of data, just insane.
