To be able to receive faxes at multiple email addresses you will need to log in to your Nextiva vFAX account. (link to How to login to your Nextiva vFAX account)
Once logged in to your vFAX account and on the main page, click Settings from the top menu.
To receive notifications at an alternate email address, click Notifications.
You can add up to 5 additional email addresses where you would like incoming faxes to be sent. You can opt for these email addresses to receive notifications on sent, received or both.