You can add one more email address by appending each email with a ',' character e.g. '[email protected],[email protected]'
To change the manufacturer email address do the following:
Navigate to the admin of Sign Customiser for your store
Click on "Customisers" in the home menu
Click on the customiser you wish to edit
Click on "Manufacturer email" in the left-hand side menu
Add the email under Receiver Email and click Save on the bottom right to save the changes.