Under settings, although wp_mail() supports multiple email 'to' addresses, it sanitizes the email addresses, removing commas.
I edited your frontend_uploader_settings.php file, and removed
'sanitize_callback' => 'sanitize_email',
which let me put anything in the email field, and I can confirm it sends to multiple (comma separated) emails. Could you add this (multiple emails) to the plugin, with a hint on the settings page to that effect?