How to generate multiple excel files within a for each loop in VB.NET

exception:
Server Error in '/' Application.
Session state has created a session id, but cannot save it because the response was already flushed by the application.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Web.HttpException: Session state has created a session id, but cannot save it because the response was already flushed by the application.
Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[HttpException (0x80004005): Session state has created a session id, but cannot save it because the response was already flushed by the application.]
System.Web.SessionState.SessionIDManager.SaveSessionID(HttpContext context, String id, Boolean& redirected, Boolean& cookieAdded) +3212120
System.Web.SessionState.SessionStateModule.CreateSessionId() +54
System.Web.SessionState.SessionStateModule.DelayedGetSessionId() +80
System.Web.SessionState.SessionStateModule.ReleaseStateGetSessionID() +19
System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs) +545
System.Web.SessionState.SessionStateModule.OnEndRequest(Object source, EventArgs eventArgs) +9818638
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +141
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69



Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.6.79.0

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        gvDataBind() 'Binding data for 1st grid
        gvDataBind1() 'Binding data for 2nd grid
        download()

    End Sub

Private Sub download()
        Response.Clear()
        Response.Buffer = True
        For Each objControl As Control In Me.Controls
            If TypeOf (objControl) Is HtmlGenericControl Then
                Dim cntrl As String = objControl.ClientID

                Select Case cntrl
                    Case "divExport" '1st div that has to be exported
                        Download1()
                    Case "divExport1"  '2nd div that has to be exported
                        Download2()
                End Select
            End If
        Next
        Response.Flush()
        Response.[End]()
    End Sub
Private Sub Download1()
        Try
            tddatereport.InnerText = "Date" + ":" + DateTime.Now.AddDays(-1).Date().ToShortDateString()
            Response.AddHeader("content-disposition", "attachment; filename= Report1 For " + DateTime.Now.AddDays(-1).ToShortDateString() + ".xls")
            Response.Charset = ""
            Response.ContentType = "application/vnd.ms-excel"
            Using sw As New StringWriter()
                Dim hw As New HtmlTextWriter(sw)
                gv1.AllowPaging = False
                gv1.AllowSorting = False
                For i As Integer = 0 To gv1.Columns.Count - 1
                    If gv1.Columns(i).HeaderText = "" Then
                        gv1.Columns.RemoveAt(i)
                    End If
                Next
                divExport.RenderControl(hw)
                'style to format numbers to string
                Dim style As String = "<style> .textmode { } </style>"
                Response.Write(style)
                Response.Output.Write(sw.ToString())
            End Using

        Catch lException As System.Threading.ThreadAbortException
            Throw lException
        Catch ex As Exception
            Throw ex
        End Try
    End Sub

Private Sub Download2()
        Try
            tddatereport1.InnerText = "Date" + ":" + DateTime.Now.AddDays(-1).Date().ToShortDateString()
            Response.AddHeader("content-disposition", "attachment; filename= Report2 For " + DateTime.Now.AddDays(-1).ToShortDateString() + ".xls")
            Response.Charset = ""
            Response.ContentType = "application/vnd.ms-excel"
            Using sw As New StringWriter()
                Dim hw As New HtmlTextWriter(sw)
                gv2.AllowPaging = False
                gv2.AllowSorting = False
                For i As Integer = 0 To gv2.Columns.Count - 1
                    If gv2.Columns(i).HeaderText = "" Then
                        gv2.Columns.RemoveAt(i)
                    End If
                Next
                divExport1.RenderControl(hw)
                'style to format numbers to string
                Dim style As String = "<style> .textmode { } </style>"
                Response.Write(style)
                Response.Output.Write(sw.ToString())
            End Using

        Catch lException As System.Threading.ThreadAbortException
            Throw lException
        Catch ex As Exception
            Throw ex
        End Try
    End Sub