How to use FaceTime with multiple devices
The email address you add have to associated with your Apple ID. You can add additional email accounts by going to My Apple ID and then click on “Manage Your Account.” After you sign in, click on “Add Email Address” and enter your new email address. You will need to verify the email account after its been added.
On your iOS device you can go to Settings-> FaceTime-> Add Another Email-> enter the email address (you will need to be signed in with your Apple ID.)
After the new email address has been added to your iOS device, you can delete the default email or leave it to be reached at multiple email accounts for video calls.