It depends on what your goal is. We keep track of parent accounts by email address to make sure they're unique. If the goal is just to allow both of you to manage the account, you can simply share the login username and password.
However, if your goal is to manage contacts and have unique names for the same phone numbers, right now, you would need more than one parent account. The reason is because contacts are currently shared across all phones. We have plans on our roadmap to update this to make it more flexible but don't have an ETA on when this will happen.