SSRS and PowerBI Report Server on same server
I now have this working on my Win-2012 server with PBIRS (August 2018) and SSRS 2016. 

First they need to have thier own ReportServer Databases. I used:


SSRS: ReportServer
PBIRS:  PBIReportServer


The Webservice URL's use the same port number but different Virtual directories. 


SSRS: http://ServerName:80/ReportServer 
PBIRS:  http://ServerName:80/PBIReportServer


And the Webportal again uses the same port number, but different URLS.


SSRS: http://ServerName:80/Reports 
PBIRS:  http://ServerName:80/PBIReports


And so far it seems to be behaving. I've restarted the server and both services started automatically and appear to be working.