Run asp.net required field validation only if at least one of the fields in the group is completed - vb.net

Question

I have a form on an asp.net (VB) page which has 2 sections.
All of section 1 is always required, but section 2 is optional. However, if section 2 is started, then all fields are required.
I'm stuck as to the logic I need to create this validation.
Here's my code:
<form runat="server">
<div>
<asp:TextBox runat="server" ID="field1a" Text="Name" />
<asp:TextBox runat="server" ID="field1b" Text="City" />
<asp:RequiredFieldValidator runat="server" ID="field1aVal" Text="Name Required" ControlToValidate="field1a" InitialValue="Name" />
<asp:RequiredFieldValidator runat="server" ID="field1bVal" Text="City Required" ControlToValidate="field1b" InitialValue="City" />
</div>
<div>
<asp:TextBox runat="server" ID="field2a" Text="Name" />
<asp:TextBox runat="server" ID="field2b" Text="City" />
<asp:RequiredFieldValidator runat="server" ID="field2aVal" Text="Name Required" ControlToValidate="field2a" InitialValue="Name" />
<asp:RequiredFieldValidator runat="server" ID="field2bVal" Text="City Required" ControlToValidate="field2b" InitialValue="City" />
</div>
<asp:button runat="server" ID="btnSubmit" Text="Submit" OnClick="submitForm" />
</form>

Answer 1

Id say use required fields on section 1, but for section two use a custom validator. Something like this.
<asp:CustomValidator ID="CustomValidator2" runat="server"
Display="Dynamic" EnableClientScript="False"
ErrorMessage="You must select one checkbox or click all offices. " OnServerValidate="OfficeVaildator">
</asp:CustomValidator>
Then in code behind do your logic
Something like this
Sub OfficeVaildator(ByVal source As Object, ByVal args As ServerValidateEventArgs)
If OfficeCheckBoxs.Checked Then
args.IsValid = True
Else
args.IsValid = False
End If
End Sub
Obviously that is really simple. You would want to check if the user altered section 2 in anyway, and if he did make him complete all fields to return your server validation arguments true. If he didnt alter section 2 or give it any data right a method that checks that and set your validation to true to allow him to proceed.