In the Rails guides (Action Mailer Basics) it says the following regarding multiple emails:
The list of emails can be an array of email addresses or a single string with the addresses separated by commas.
So both "test1@gmail.com, test1@gmail.com" and ["test1@gmail.com", "test1@gmail.com"] should work.
See more at: http://guides.rubyonrails.org/action_mailer_basics.html