When sending a message, check to see if it lets you add a second name. There is a way to filter for Observers, which could be how you can add the second name. Check out this guide: https://community.canvaslms.com/t5/Observer-Guide/How-do-I-send-a-message-to-a-course-instructor-in-...
I am not sure if Observers have the ability to include other Observers when sending a message to anyone through Canvas. This is likely due to privacy protection. In this case, standard emails would likely work better. The Canvas Inbox is not intended to be a fully functional email system. So if the above method does not allow you to select a second person, consider an email instead.
