Are multiple defragmenters a good idea?

"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."

"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."""