var
...
  dsMaster: TDataSource;
...
begin
...
  mtFiles := TFDMemTable.Create(nil);
  try
    mtFiles.CloneCursor(FDFiles);
    dsMaster := TDataSource.Create(nil);
    try
      dsMaster.DataSet := mtFiles;
      mtMetering := TFDMemTable.Create(nil);
      try
        mtMetering.MasterSource := dsMaster;
        mtMetering.MasterFields :=
        mtMetering.DetailFields := 
        mtMetering.CloneCursor(FDMetering,True,True);
        ...
      finally
        FreeAndNil(mtMetering);
      end;
    finally
      FreeAndNil(dsMaster);
    end;
  finally
    FreeAndNil(mtFiles);
  end;
...
end