Multi-brand help centers and default landing page

We have five brands in Zendesk Professional. We have a unique help center for each brand. However, agents across all brands land on the default brand for the Zendesk Professional account. I'm looking for a way to have Brand A agents default to the Brand A help center when they open the guide; and Brand B agents default to Brand B help center, etc. I've been in touch with Zendesk Support and they do not have a way to do this.

This is hurting our business because Brand A information is not accurate for Brand B customers and our agents can become easily confused about which brand help center they are looking at. We are increasing our internal training as a workaround, but this is time-consuming and expensive. Is there a way that Zendesk could allow for default landing pages for each brand, instead of the entire account? 

Has anyone else had this problem and found a workaround?