The additional host is intended to be applied to the account itself not the individual listing. However the process of adding an additional host is via individual listings and appears to be restricting application to only one listing; a contradiction of the Airbnb's additional host supporting statement ' Following extensive research into a number of ways we can help, we’ll be introducing a new feature that allows current hosts to add an additional host to their account. This is a simple way to allow a family member or trusted friend to help with some of the hosting responsibilities, without giving someone access to their personal account.'.
I would suggest you contact them directly to discuss this. They may be able to override their system from their end in order that you can be linked to the account as opposed to just one listing. This is a new feature so this is possibly one of the teething problems. Click on link for contact details - Contact Airbnb
It would be appreciated of you update this conversation with the outcome just to give us an idea of how to overcome this situation or if we just have to live with it for now.....
Best wishes
Ama