PivotGridSpreadExport and FileExportMode = FileExportMode.NewSheetInExistingFile

         List<Tuple<int, string, bool>> jurisdictionTypes =
            new JurisdictionTypeDA().GetJurisdictionTypeKeyValueList();

         foreach (Tuple<int, string, bool> jurisdictionType in jurisdictionTypes)
         {
            RadPivotGrid tempPivotExportGrid = new RadPivotGrid
            {
               ColumnGrandTotalsPosition = TotalsPos.None,
               ColumnsSubTotalsPosition = TotalsPos.None,
               EmptyValueString = "0",
               RowGrandTotalsPosition = TotalsPos.None,
               RowsSubTotalsPosition = TotalsPos.None
            };

            var exportResults = _apportionmentInfoPivot
               .Where(app => app.JurisdictionTypeKey == jurisdictionType.Item1
               .OrderBy(app => app.PartySort)
               .ThenBy(app => app.LegacyKeyValue)
               .Select(app => new
               {
                  jurisdiction_name = app.JurisdictionName?.Trim() ?? string.Empty,
                  party_name = app.PartyName?.Trim() ?? string.Empty,
                  totalvoters = app.TotalVoters,
                  signaturesneeded = app.SignaturesNeeded
               })
               .ToList();

            tempPivotExportGrid.DataSource = exportResults;
            tempPivotExportGrid.RowGroupDescriptions.Clear();
            tempPivotExportGrid.ColumnGroupDescriptions.Clear();
            tempPivotExportGrid.AggregateDescriptions.Clear();

            // Row Information
            PropertyGroupDescription myRows = new PropertyGroupDescription
            {
               PropertyName = "jurisdiction_name",
               CustomName = "Jurisdiction",
               SortOrder = Telerik.Pivot.Core.SortOrder.None
            };

            tempPivotExportGrid.RowGroupDescriptions.Add(myRows);

            // Column Information
            PropertyGroupDescription myCols = new PropertyGroupDescription
            {
               PropertyName = "party_name",
               CustomName = "Party",
               SortOrder = Telerik.Pivot.Core.SortOrder.None
            };
            tempPivotExportGrid.ColumnGroupDescriptions.Add(myCols);

            // Aggregate Information
            tempPivotExportGrid.AggregateDescriptions.Add(new PropertyAggregateDescription()
            {
               PropertyName = "totalvoters",
               CustomName = "Count",
               AggregateFunction = AggregateFunctions.Sum,
               StringFormat = "#,##0"
            });

            tempPivotExportGrid.AggregateDescriptions.Add(new PropertyAggregateDescription()
            {
               PropertyName = "signaturesneeded",
               CustomName = "Needed",
               AggregateFunction = AggregateFunctions.Sum,
               StringFormat = "#,##0"
            });

            tempPivotExportGrid.AggregatesPosition = PivotAxis.Columns;

            // Combine each result into a new tab
            PivotGridSpreadExport spreadExport = new PivotGridSpreadExport(tempPivotExportGrid)
            {
               SheetName = jurisdictionType.Item2,
               FileExportMode = FileExportMode.NewSheetInExistingFile
            };
            spreadExport.RunExport(myExportFile, new SpreadExportRenderer());

            // Send each result to their own sheet
            PivotGridSpreadExport mySpreadExport2 = new PivotGridSpreadExport(tempPivotExportGrid)
            {
               SheetName = jurisdictionType.Item2,
               FileExportMode = FileExportMode.CreateOrOverrideFile
            };
            string newfile = string.Concat(Path.Combine(Path.GetDirectoryName(myExportFile) ?? string.Empty, Path.GetFileNameWithoutExtension(myExportFile)), "_", jurisdictionType.Item2, Path.GetExtension(myExportFile));
            mySpreadExport2.RunExport(newfile, new SpreadExportRenderer());


         }