How to login to app on partners phone with Duo account?

Welcome to the forum @Ian-Andrea

There was a thread about duo membership where other members had the same issue

At the time it was passed to member services but they didn’t come back with a solution

The work around suggested by forum members was to create a shared email address so your husband and you can both access the verification code

https://forum.trustedhousesitters.com/t/constant-logging-into-the-app-with-verification-code-needed