Apple ID

A phone number can only be associated with one Apple ID at a time. If you sign in to your husband's Apple ID, data from his device would sync to your device and your data would sync to his device. If you have to monitor his Messages, add his email only to Messages/FaceTime. You can add the email as an additional email address on your Apple ID.

If you were to add his phone number to your Apple ID, then messages sent to him (to his number) would also go to your phone, as it would be treated as one of "his" devices. He would not be able to see your messages, as far as I know.

So, in short- yes, it would probably work.

The email might work, but by adding his email, you're going to get any message related to his Apple ID on your iPhone. I've never done this before, so I can't say for sure of the consequences.

It's worth noting, however, that intercepting his messages without his knowledge and consent is illegal in the US (as per 18 U.S. Code § 2511).

iMessages are end-to-end encrypted.

They can ONLY be read on the Trusted Devices of the AppleIDs of the “sender” and intended “receiver.”

They are intentionally designed to PREVENT interception in-transit and reading by anyone else.

I don't think it would work that way. You should never share Apple IDs, as that causes a whole host of issues - Make sure that each family member has a unique Apple ID - Apple Support

I’d suggest trying what Eric proposed.

My post was only to provide additional background info; which might also help explain any unexpected behavior you might encounter going-forward.