Can I use
one extension times two times? Like below, as when when I use ACCSV, it exports
in .CSV, i want it in .txt extension
Yes, you can add two extensions. You can add as many extensions as there are formats supported by Aspose.Cells for Reporting Services.
Also, the extensions names should not be same, they should be different. I have highlighted the important things with red color.
Please try following update in rsreportserver.config