FaceTime calls ringing on multiple phones

You are saying that both of his devices ring at the same time, even when you call regularly? If so, then on his iPhone, check Settings>Phone>Calls on Other Devices, see if that is turned on. If so, turn it off. Check on both phones. Also in Settings>FaceTime>You Can Be Reached By FaceTime At, and see which numbers are checked. Do this on both devices. Make sure only the actual device number is checked.

Permanently change settings: Settings > Apple ID > Name, Phone Numbers, Reachable At > add or remoce as appropriate.

Temporarily change settings: Settings > Messages (Or FaceTime) > check or uncheck the Reachable At entries in the list

So there are no common email addresses listed across the (for lack of a better differentiation) home and office iPhones?

And I’m here assuming the home and work iPhones have separate Apple IDs. If home and work are using the same Apple ID, this is expected. (That can be disabled, by disabling what Apple calls Continuity.)

Is the arriving FaceTime call a group call?

If there are no common email addresses, and there are no common phone numbers in the Reachable At list? If not, and if these home and work devices are not associated with a common Apple ID, and these are not group calls, then I’m out of ideas.