Validatoron checkbox list based on checkbox

I’m trying to build an email form that has some validators. 
The form has a textbox field (txtLocalName) and a checkbox list (cblScope) that will only appear when cbMultistate is checked.
I got the txtLocalName validator to work, but I cannot figure out how to validate that checkbox list only when cbMultistate is checked and the checkboxlist is visible. I assume I need to use a custom validator, but having it only enabled when cbMultistate is checked is throwing me off.
<table border="0" cellpadding="5" cellspacing="0" class="normal"> <tr> <td>Email address of Sales Rep:</td> <td><asp:TextBox ID="txtToEmail" runat="server"></asp:TextBox></td> <td>(for multiple recipients, add a comma in between email addresses) <asp:RequiredFieldValidator ID=" ...