Exporting multiple grids to one excel doc

grid export function

WINFORMS
    gridview ver: 2019.3.917.40

What happens is the excel doc ends up with only the one sheet, the 2nd one.
Why?
What am I doing wrong?

I send in;
     a grid Telerik.WinControls.UI.RadGridView objGrid
     SheetName
     NbrOfGrids
    
Calling code!
     strDocToOpen = exportGrid(radGrid1, "sheetname1", 2);
     strDocToOpen = exportGrid(radGrid2, "sheetname2", 2);
    
     Call to Open strDocToOpen
    

EXPORT Function code!

// export the grid to an excel file
Telerik.WinControls.Export.GridViewSpreadExport exporter = new Telerik.WinControls.Export.GridViewSpreadExport(objGrid)
{
    ExportFormat = Telerik.WinControls.Export.SpreadExportFormat.Xlsx
};
Telerik.WinControls.Export.SpreadExportRenderer exporterRenderer = new Telerik.WinControls.Export.SpreadExportRenderer();

exporter.SheetName = strSheetName;   

// if more then one grid to be in exceldoc
if (intNbrOfGrids > 1)
{
    exporter.FileExportMode = Telerik.WinControls.Export.FileExportMode.NewSheetInExistingFile;
};

string strPathToMyDocuments = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
strFileName = "MyDoc";

exporter.RunExport(strPathToMyDocuments + @"\" + strFileName + ".xlsx", exporterRenderer);
 
Returns  (strPathToMyDocuments + @"\" + strFileName + ".xlsx")