[HttpPost]
public ActionResult CompareExcel(ExcelModel model, string submitValue, string compareOption)
{

        TempData["firstFile"] = model.Files[0];
        TempData["secondFile"] = model.Files[1];

        if (submitValue == "Compare calculations")
        {
            var firstFile = TempData["firstFile"];
            var secondFile = TempData["secondFile"];
            if (firstFile == null || secondFile == null)
                return View("CompareExcel");
            else
                return CompareColumn(compareOption);
        }
        //Assume submitValue is "Compare calculations"
}

[HttpPost]
    public ActionResult CompareColumn(string compareOption)
    {
        List<Calculation> cList = new List<Calculation>();
        Calculation calc = new Calculation();
        BigModel m = new BigModel();
        HttpPostedFileBase firstFile = (HttpPostedFileBase)TempData["firstFile"];
        HttpPostedFileBase secondFile = (HttpPostedFileBase)TempData["secondFile"];
        DataSet firstSet = ExcelToDataSet.ConvertToDataSet(firstFile);
        DataSet secondSet = ExcelToDataSet.ConvertToDataSet(secondFile);
        for (var i = 0; i < firstSet.Tables[0].Rows.Count; i++) //Get Object reference not set to an instance of an object
        {
            DataRow data = firstSet.Tables[0].Rows[i];
            calc.PresentValue = Convert.ToDecimal(data.Table.Rows[i]["Capital balance"]);
        }

        return View();
    }

public partial class GetExcel
{

    public List<HttpPostedFileBase> Files { get; set; }

    public GetExcel()
    {
        Files = new List<HttpPostedFileBase>();
    }
}

public static DataSet ConvertToDataSet(HttpPostedFileBase excelFile)
    {
        DataSet result = null;

        if (excelFile != null && excelFile.ContentLength > 0)
        {
            // .xlsx
            IExcelDataReader reader = ExcelReaderFactory.CreateOpenXmlReader(excelFile.InputStream);

            // .xls
            //IExcelDataReader reader = ExcelReaderFactory.CreateBinaryReader(file.InputStream);

            reader.IsFirstRowAsColumnNames = true; // if your first row contains column names
            result = reader.AsDataSet();
            reader.Close();
        }
        return result;
    }