How to get values of associated contrl's

Question

I have created a web user control for numeric pad to enter numbers for touch screen application.
<asp:Panel ID="pnlPopUp" runat="server">
  <table>
  <tr>
  <td>
  <asp:Label ID="lblNumbers" runat="server" Text="Numbers:" />
  </td>
  <td>
  <asp:TextBox ID="txtNumbers" runat="server" />
  </td>
  </tr>
  <tr>
  ...rows for numeric buttons...
  </tr>
  <tr>
  <td colspan="2">
  <asp:Button ID="btnOK" OnClick="btnOK_Click" runat="server" Text="OK" />
  <asp:Button ID="btnClose" runat="server" Text="Close" />
  </td>
  </tr>
  </table>
</asp:Panel>
<asp:ModalPopupExtender ID="popupPnl" runat="server" DynamicServicePath="" Enabled="True" TargetControlID="btnShow" CancelControlID="btnClose" PopupControlID="pnlPopUp">
</asp:ModalPopupExtender>
And created a method to show popUp, an event to get Ok button click event and a property to get entered text.
  public void ShowPopUp()
  {
  popupPnl.Show();
  }
 
  string textBoxValue = "";
  public string txtValue
  {
  get
  {
  return textBoxValue;
  }
  }
  public event EventHandler OKButtonClick;
  public void btnOK_Click(object sender, EventArgs e)
  {
  textBoxValue = txtNumbers.Text;
  OKButtonClick(sender, e);
  txtNumbers.Text = "";
  }
And then added an aspx page and register the userControl
    Numbers:
  <asp:TextBox ID="txtNum" runat="server" />
  <asp:Button ID="button1" runat="server" Text="Show" OnClick="button1_Click" />
  <br />
   Numbers2:
  <asp:TextBox ID="txtNum2" runat="server" />
  <asp:Button ID="button2" runat="server" Text="Show" OnClick="button1_Click" />
  <uc1:WebUserControl ID="popUp" runat="server" />
aspx.cs
  protected void Page_Load(object sender, EventArgs e)
  {
  popUp.OKButtonClick += new EventHandler(Ok_Click);
  }
 
  protected void button1_Click(object sender, EventArgs e)
  {
  popUp.ShowPopUp();
  }
 
  protected void Ok_Click(object sender, EventArgs e)
  {
  txtNum.Text = popUp.txtValue;
  }
It's working fine. Now when I click the button1 I want to add values of TextBox of popUp control's in txtNum or I click the button2 then I want to add values of TextBox of popUp control's in txtNum2 after clicking the btnOK of popUp control.