Join stored proc results from linked servers into SSRS drill down report

Currently I have a 2015 SSRS report calling a stored procedure that passes parameters to drill through report, but soon my database will to be split in two separate servers. All columns, tables, etc will remain the same in both servers, only the data will split. How can I join the results from both servers into one in SSRS report preserving the drill through reports? I will have 3 servers configured like this: server 1 is the current server with all the employee data but starting next week sales employee data will no longer be loaded into this server. server 2 will be the new server containing the only the new sales employee data. the old sales employee data remains in the old server but still needs to be included in the report. server 3 will have a link server to both server 1 and server 2. All SQL servers are 2016