I have tried 2 things- first I tried to add a data source to an existing entity in my extension. However the context menu does't have an option to allow me to add a 2nd table as a data source.

Second, I tried to create a new entity from scratch. I am able to set the primary data source but unable to add more data sources. The option to 'add data source' is there but greyed out.

How can I add multiple data sources to either a newly created entity or an existing entity I am extending?