Hi Cao,
Thank you for contacting Aspose support.
First of all, please note that Excel application allows to select multiple worksheet at a time, however, the active worksheet is only one at all times. Please check the attached snapshot for elaboration. Aspose.Cells APIs behave the same way, that is; you can select multiple worksheet while using the Worksheet.IsSelected property whereas the WorksheetCollection.ActiveSheetIndex property will always return single index because only one worksheet can be active at a time.
Please check the following piece of code to select multiple worksheets.
C#
var book = new Workbook(@“book1.xlsx”);
book.Worksheets[0].IsSelected = true;
book.Worksheets[1].IsSelected = true;
book.Save(@“output.xlsx”);