If you have more than one person that you would like to assign as an editor to a proof, you can.
To assign an editor
When the proof has been returned with a to-do list for actioning, open the proof and click send to editor.
Add the editor’s email address (they can be inside or outside of your email domain) and write a message to the editor if you would like. Then click send to-dos.
To assign additional editors
Open the proof’s info pane by clicking the i icon.
Click the plus icon to the right of the editor to add additional editors.