GNU gettext msgfmt can merge several po files into one single mo file.
The current version of msgfmt can only compile one po file to one mo file.

After looking at the code, the enhancement should be simple to implement.

Command line: if one output file is given (option -o) and several input files, then all the input files should be combined.

Implementation:
- main should pass all the parameters to make (*args)
- make should accept one single string for compatibility or an iterable of string. In that latter case, the current processing should be repeated on all input files.

I could propose a patch (but I am afraid it ends being rather large) or a pull request. As a new user here, I do not know what is the best way...