Two text domains in one plugin

Yes, you can load multiple translation files for multiple text domains in one plugin. This is sometimes useful, for example when you need just one string in AJAX calls and many strings in the rest of the application: to make AJAX faster, load a separate translation.

Be aware, automatic language updates for the plugin from wordpress.org will probably not work with that, they don’t expect more than one translation.

Never ever use plugins_loaded to load a translation. You don’t create output here. Wait for the first call to a translation function, make sure you don’t load the translation when it isn’t needed. The comment post file, XML RPC or AJAX are common places where plugins slow down everything without any benefit.