the tooltip for the site property says you can't reference all sites: "Defines the website (specified by its code name) from which to load the content. If left empty, the current site is used."
You could aggregate the data using a query:
- Add a Query Data Source web part
- Create a new query SELECT ##COLUMNS## FROM select * from View_CONTENT_Article_Joined WHERE ##WHERE## for the webpart
- Add the columns you want to return in the columns field on the webpart
- Add the where condition to the webpart field: nodesiteid = x or nodesiteid = y
- Add basic repeater
- set the datasource name of the basic repeater to the datasource created in step 1