Wave should have an option to have two or more owners, If one owner creates the account the other owner can only be added as a highest function of admin. If they both use the bank account and need to work on the accounting only one has the option to bank access.
At this time, Wave does not have a feature which allows an account to have more that one account owner but I really appreciate you reaching out and letting us know that this is something you would be interested in. As such, I have moved your comment to our Account Settings Feature Ideas forum so that our developers are able to view and consider your feature idea in the future.
Feel free to let us know more about how this feature would be useful for you!