passing more than one variable as parameter value to ssrs by ssis

public Sub Main()

        Dim url, destination As String
        destination = Dts.Variables("report_destination").Value.ToString + "\" + "Report_" + Format(Now, "yyyyMMdd") + ".xls"
        url = "http://localhost/ReportServer?/ssis_resport_execution/ssis_ssrs_report&rs:Command=Render&ProductID=" + Dts.Variables("ProductID").Value.ToString + "&user_id" + Dts.Variables("user_id").Value.ToString+ "&rs:Format=EXCEL"
        SaveFile(url, destination)
        Dts.TaskResult = ScriptResults.Success
    End Sub

how to pass more than one variable values in ssis as parameter values to ssrs.

with the above code its showing as empty.If i am taking single variable i am able to render the data into  excel sheet.

can any one suggest on the above issue.