It is a two step process:
1. Open your KML/KMZ files and export them as a CSV.
2. Merge multiple CSV files with e.g. Notepad/TextEditor
3. Convert your new CSV to KML file and save it as a KML file, or as a Garmin Map file for Android phones
This process is safe, online, nothing to install. It takes about 10-15 seconds per file.
Couldn't get it to merge GMaps KMLs. Similar work-around.
I'm merging KML files for Google Maps. Basically I mapped out the extensive rail system of a nearby city so it would always be obvious when traveling where I was relative to stations and I wouldn't have to muddle around with zooming on poor data connections. It should also good for giving people directions when Google Maps isn't cooperating because I can easily tell where nearby stations were in certain districts and where nearby rail-lines connected (these aren't marked in maps).
The problem? Well the map was created with the intention of being used traveling, so rail-lines were only one of the things I wanted to be shown. However I didn't realize Google Maps' free version would only allow me to have 3 layers at a time, and there was no way to copy the details of one layer to another. Additionally when importing KML files, once a file has been imported to a layer, no other files can be imported to that same layer. So I ate through all 3 layers just seeing the transit, and not all the events nearby, with no way of making all that info one layer. Now I could start adding the other items to one of the railways but this would be really clunky.
Solution? Google it. Found this but what you suggested didn't work for me. When I used (only tried a couple) of these services to import my kml files and export them as csv files their size would reduce dramatically. When opening those csv files I got to see why. For some reason (maybe this only happens with GMaps kmls) the data points were lost in the conversion and only the headers remained. Merging these failed as might be expected. However, I found that if you open the straight kml files in text edit they will properly import as a single kml file IF you merge the kml and document sections of each kml so you only have one set of each tag in the kml file.
So instead of:
Where ~blah is whatever stuff is in-between those broad tags.
Sorry for being long-winded, maybe this will help someone down the road. Thanks for getting me on the right track.