I am glad that this post was useful 
Well, communicating between the two models should not be much harded that with only one (https://github.com/mailboxer/mailboxer#preparing-your-models). What will cetrainly change is this line https://github.com/bodrovis/SitePoint-Mailboxer/blob/master/app/controllers/messages_controller.rb#L8 because you’ll need to add support for another model. But all in all most of the codebase will stay intact. For example, this helper https://github.com/bodrovis/SitePoint-Mailboxer/blob/master/app/helpers/messages_helper.rb#L4 should be changed as well (I am not sure if you want to present both users and biz_users in that list).
So there is no need to create additional controllers - those two are perfectly fit for working with messages and conversations.