How do coordinators assign two proctors to a room or proctor a room themselves?

You can’t assign more than one proctor to a room in Test Day Toolkit, but you don’t need to.  

Anyone who can access the administration in Test Day Toolkit, including coordinators, can proctor a room, even if they aren’t assigned the proctor role or assigned to the room. They can just navigate from the All Rooms page to the room’s welcome screen. From there they can take attendance, read the script, share the codes, and monitor student progress.

If you want to proctor a room and no one else is assigned to it, assign yourself to the room but you don’t need to change your role.