You can email to the second address, but only after you’ve created the email in the first place. Then you can click on the person’s name and add the second email to the email, then delete the first email address. We’ve found no other way to do it and there is no way to “email all email addresses for this contact” that we’re aware of.
We’re happy enough with the multiple email field as is, but there’s so much else that’s wrong with HubSpot these days that we’ve moved back to Excel and Trello. I guess the shine is gone.