Visual Basic - Save form in txt with format

The contact so they can help me with a problem that I have in a small form that I use with visual basic to read data.

I have a form with two fields: row and item number I need to save in the following way:

<table>
<th>fila</th>
<th>item</th>
<tbody>
<tr><td>f1</td><td>1</td></tr>
<tr><td>f1</td><td>2</td></tr>
<tr><td>f1</td><td>3</td></tr>
<tr><td>f1</td><td>4</td></tr>

</tbody>
</table>

The row (txtfila) is written first and when I press enter the item number (txtpalet) of that row can be entered, which can be many. When I finish inserting the item numbers for that row I press the save button and generate a .csv file on the computer.

This works, but I can only save the item number:

item 
1
2
3
4
5
6

I can not save another column to that file that is the row ...

This is my code:

Imports System.IO
Imports System.Text

Public Class Form1
    Dim _stringBuilder As StringBuilder

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
        txtFila.Focus()
        _stringBuilder = New StringBuilder()
    End Sub

    Private Sub txtPalet_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtPalet.KeyPress

        If e.KeyChar = ChrW(Keys.Enter) Then
            _stringBuilder.Append(txtFila.Text)
            _stringBuilder.Append(txtPalet.Text).AppendLine()

            txtPalet.Text = ""
        End If
    End Sub


    Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim fecha As String
        Dim hora As String
        Dim archivo As String

        fecha = Format(Now, "dd-mm-yyyy")
        hora = Now.ToString("HHmm")
        archivo = "bpt1" + "_" + fecha + "_" + hora + ".csv"

        Dim writer = New StreamWriter(archivo, True)
        writer.Write(_stringBuilder.ToString())
        writer.Flush()
        writer.Close()
        MsgBox("Documento Guardado como : " + archivo)
    End Sub

    Private Sub txtFila_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtFila.KeyPress
        Dim valorFila As String

        If e.KeyChar = ChrW(Keys.Enter) Then
            valorFila = txtFila.Text
            If Len(valorFila) >= 2 Then
                txtPalet.Focus()
                txtFila.ReadOnly = True
            Else
                MsgBox("Ingrese una Fila Valida")
                txtFila.Text = ""
                txtFila.Focus()
            End If
        End If
    End Sub
End Class

If you can give me the information on how to include the new column in the file, have:

column 1 (row) column 2 (item)

thanks.