The q2a sitemap is not saved on the file system but is generated on the fly when you request it ( http://www.yourq2asite.com/sitemap.xml ).
It's dinamically generated by the xml-sitemap-plugin, that is a q2a standard plugin.
If you want to change for some reason (that I've not understood. Do you want to use 2 sitemap.xml ?) the url at which this plugin "listens" for the request, you should change the behaviour of the plugin.
See this line :
you could change it for instance
so that you can now invoke the plugin at the new url :
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May 7, 2020: Q2A 1.8.4 released!