Linked reports with Business Units can be created from Pre-Defined Reports with Business Units or a normal report marked with Export drilldown for Business Unit.
Read here about, what is a linked report
You can Deploy one or more of the Pre-Defiend reports for Business unit to Reporting Services. Deploy them to the Business Unit Template folder.
You can also Create a report, and when deplying to Reporting Services, Enable drill down and Export drilldown for Business unit. Deploy the report to the Business Unit Template folder.
This will create a property for the Business Unit, which is needed for using it with Linked Reports.
Now go to Create linked reports