I think it is great that you are so sensitive to the propriety of having your partner's email remain private, but I think maybe you should look at it in a slightly different way.
If this is your Dreamhost account, it is entirely appropriate that you have the ability to determine the passwords of users whose activities on the account are your responsibility.
In a shared environment, this is not unusual. Having the ability to manage the account, and retrieve the passwords of your users, is important. None of that, however, means you have to use that ability to read another's email; I maintain records of all my clients' passwords - but I do not read their mail, as I feel that to do so is ethically wrong.
You might consider that if your relationship with your partner is not sufficiently trusting that you can simply assure him/her that you would never abuse your ability, as the account manager, by using their password, and have that be sufficient, then maybe the partnership is ill-advised.
You having access to his email password only prevents his email form being "private" if you abuse that knowledge.
All that said, I do not know of a way for you to maintain your status as the "master" account holder without having your "guest" users credentials available to you via the Panel.