static void Main(string[] args) {

    Excel.Application app = new Excel.Application();

    Excel._Workbook result = app.Workbooks.Add();

    Excel._Workbook wb1 = app.Workbooks.Open(Path.GetFullPath("./Sources/Source1.xlsx"));

    Excel._Workbook wb2 = app.Workbooks.Open(Path.GetFullPath("./Sources/Source2.xlsx"));

    foreach (Excel._Worksheet each in wb1.Sheets) {

        each.Copy(result.Worksheets[]);

    }

    foreach (Excel._Worksheet each in wb2.Sheets) {

        each.Copy(result.Worksheets[]);

    }

    result.SaveAs(@"c:\result.xlsx");

    Console.WriteLine("press any key to exit.");

    Console.ReadLine();

    wb1.Close();

    wb2.Close();

    app.Quit();

}