You can also try gpxstudio for merge.
Help: gpx.studio — help | Merge
Page/Webapp: gpx.studio — app
- Menu > File > Open > Mark all files > open [required files by holding Shift-Key or CTRL-Key and mark files]
- Menu > Edit > Select all
- Left Menu > Merge > Selection as in picture
- Menu > File > Export
Then import as overlay in Kurviger. My little test worked fine.