RequiredValidation only active when another control contain a value - asp.net

Question

I have two dropdownlists, if one of them contain a value (not default value, which is empty string), the other should also have a value (like requredvalidator). But if no one has a value the page should validate 'true'.
I cannot solve it with a validation group, because the button that trigger the validation is already triggering other validations.

Answer 1

Could you just use a custom validator control and put your logic in the server side validation method. Keep it in the same validation group.
<asp:CustomValidator ID="valCust" runat="server"
ControlToValidate="ddlControl" ErrorMessage="error Message"
ValidationGroup="Group"
onservervalidate="valCust_ServerValidate" >*</asp:CustomValidator>
code behind
protected void valCust_ServerValidate(object source, ServerValidateEventArgs args)
{
args.IsValid = MethodToWorkOutValidation();
}

Answer 2

please check below code snippet.
function EnableDisableValidator() {
var DropDownList1 = document.getElementById('DropDownList1');
var DropDownList2 = document.getElementById('DropDownList2');
var RequiredFieldValidator12 = document.getElementById('RequiredFieldValidator12');
if (DropDownList1.selectedIndex > 0 && DropDownList2.selectedIndex > 0) {
ValidatorEnable(document.getElementById('RequiredFieldValidator12'), true);
}
else {
ValidatorEnable(document.getElementById('RequiredFieldValidator12'), false);
}
}
..........
<asp:DropDownList ID="DropDownList1" runat="server" onchange="EnableDisableValidator();">
<asp:ListItem Text="" Value=""></asp:ListItem>
<asp:ListItem Text="1" Value="1"></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownList2" runat="server" onchange="EnableDisableValidator();">
<asp:ListItem Text="" Value=""></asp:ListItem>
<asp:ListItem Text="1" Value="1"></asp:ListItem>
</asp:DropDownList>
<div style="display: none;">
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</div>
<asp:RequiredFieldValidator ID="RequiredFieldValidator12" runat="server" ControlToValidate="TextBox1"
Enabled="false" ErrorMessage="*************" ></asp:RequiredFieldValidator>
Let me know if any concern