Hi,
merge function you found is correct method to do this. Anyway order may be a problem.
What you see in app is name took from "name" defined directly in the file. Filename is not important here in case, name is defined in file. Open any GPX file in any text editor (may recommend Notepad++) and you will there see "<name>track name</name>" field. This needs to be changed, not the name of file.