How do you handle multiple feed URLs in a migration?
You should be able to list any URLs that should be fetched:

source:
  plugin: url
  data_fetcher_plugin: http    
  data_parser_plugin: xml   
  urls:
    - http://myfeed.com/newsfeed/get.php?id=123456
    - http://myfeed.com/newsfeed/get.php?id=123457
    - http://myfeed.com/newsfeed/get.php?id=123458


The Url plugin in migrate_plus converts single url values into an array by default: https://git.drupalcode.org/project/migrate_plus/blob/HEAD/src/Plugin/migrate/source/Url.php#L34