We do it this way:
First create an app and make sure we have all the master items developed in the same.
Once done: We duplicate the sheet and work on visualizations independently.
Once we are done with the visualizations, we copy them into the master app. (A visualization can be copied between apps if both the apps have the measures and dimensions in common).
Thanks and Regards,
I have a solution, by using custom properties and security rules in order to be able to work multiple developers on one unpublished app. I have put this in my blog:
Shortly, you define Custom properties and put and object and users related to the app into it. In the security rule you define, you provide the rights. This works even for unpublished apps, which is superior.