Comments by Livalwas (Top 1 by date)

Take a look at this article. It shows how to create a workbook from the GridView control. Now what you'll need to do is create two workbooks (one for each GridView) and then add the worksheet from the second one into a first one, something like the following:

excel1.Worksheets.AddCopy("Sheet2", excel2.Worksheets[0]);

Note that this article uses this C# library for excel's workbooks.