KmlMerger

This geographic mapping data tool merges multiple Keyhole Markup Language (KML) files into a single file and layer. Currently, KMZ files are not supported but you can unzip them yourself.

The merging procedure used is equivalent to the "XML" method used on the website version of this software available on kmlmerger.com. Feel free to test it out there before downloading. The "XML" method simply combines the contents of each KML file and puts them into a single file, much like if you were to copy and paste the contents of each file into one file. This keeps styling information, however it might cause duplicate styles and elements. Typically, this is not an issue.