I use Fastmail for this. Here's what we do:

- My wife and I each have our own email addresses

- We have a third email address that we share with the school (etc.); this email address is not a real inbox but a forwarding address that sends mail to the first two

- When email is sent TO this address, the default is to reply FROM the address

- When email is sent FROM this address, an Auto BCC rule sends a copy to the other spouse

In this way we both get our own personal email addresses, but we have a shared address that goes to both of us, and we know if an email sent to that address has been replied to, what the reply was, etc.