a trouble with validate server control??

I had a server form like this:

<form runat=server>
Username <asp:textbox id='txtUN' runat='server'>

Password <asp:textbox id='txtPSW' runat='server'>

FirstName <asp:textbox id='txtFN' runat='server'>

LastName <asp:textbox id='txtLN' runat='server'>

<asp:button id='cmdSubmit' runat=server>
</form>

I want to check this form with these rule:
- force require Username & Password
- If specify FirstName -> require LastName

I try to make validate:

<form runat=server>
Username <asp:textbox id='txtUN' runat='server'>
<asp:requiredfieldvalidator ControlToValidate="txtUN" runat="server" Text=" specify username"/>

Password <asp:textbox id='txtPSW' runat='server'>
<asp:requiredfieldvalidator ControlToValidate="txtPSW" runat="server" Text=" specify password"/>

FirstName <asp:textbox id='txtFN' runat='server'>

LastName <asp:textbox id='txtLN' runat='server'>

<asp:button id='cmdSubmit' runat=server>
</form>

Anyone can help me to validate "If specify FirstName -> require LastName"??

Thanks very much!

 

I would:

1. Create a CustomValidator control.
2. Set ControToValidate as txtFN so that if text is entered it will run a custom function.
3. Create a custom client-side (javascript) function that checks whether txtLN is blank. Return 0 if blank.
4. Set ClientValidationFunction as your custom function.

 

Can you show me the detail, please...