It is not possible to have two email addresses affiliated with your AirBNB account. This doesn't mean that you can't both get notifications, however.
Option 1 is to have an email address affiliated with your account that is ONLY for AirBNB emails. Both of you would have access to this email account, and both could respond.
Option 2 is to set up a "rule" for the email account that you already use to manage AirBNB. This "rule" would look at the "from" field of all incoming email and forward anything from AirBNB to a different email account. The second email address would have to access the AirBNB platform to respond, but would be alerted to all incoming correspondence.
Does that help?