Is it possible for notifications of completed forms to be sent to two or more email addresses?

I work with several collaborators and it would be very useful if we would all receive a message when someone has submitted their form.

Yes, it is possible, please check this guide: https://www.jotform.com/help/39-Send-Notifications-to-Multiple-Recipients 

Open a new thread if you need anything else, we will be glad to assist you.