Zonmaster only supports 1 email for your notifications.
If you want to send out emails to multiple people for notifications we suggest creating a rule in your email client to forward it to multiple accounts.
If you are using Gmail what we suggest is something like this:
1. Set the notification email to be something like myname+zonmasternotifications@gmail.com
2. Create a rule in Gmail for any email coming to that address and forward it to whoever you think should get it
Gmail ignores anything after the + and it is essentially a way to "tag" incoming emails (We use this for all those annoying newsletters that you seem to have to sign up for. That way you can have some of them set to immediately archive).
This also works for any custom domain hosted on Gmail.