C# – ASP.NET How to 2 dynamically loaded user controls respond to each others events

You're asking if you can keep the control loaded in the page?

You can't keep the control loaded in the page, because the page doesn't exist! It's recreated on every request. It's created from the markup (compiled or not), and from running any code in the CodeBehind. If you want your controls to be there after a PostBack, then you need to recreate them on the PostBack. If you want to process events on those controls, then you'll have to wire the controls to event handlers when you recreate them after PostBack. Then the events will fire.

Little different version of what drs9222 had answered.
1. Declare a delegate

Public delegate void UserControlFormSubmit(object sender, EventArgs e);

2. Declare an event inside user control of type UserControlFormSubmit

Public event UserControlFormSubmit OnFormSubmit;

3. Set User control event as trigger for update panel like this

<asp:UpdatePanel ID="paymentFormUpdater" runat="server" UpdateMode=”Conditional” ChildrenAsTriggers=”true”> 
<ContentTemplate> 
    <asp:PlaceHolder runat="server" ID="plcPaymentForm" /> 
</ContentTemplate> 
<Triggers>
   <asp:AsyncPostBackTrigger ControlID="paymentHeader1"            EventName="OnFormSubmit" />

4. Raise the event OnFormSubmit when selectedindexchange event occurs for the radioButtonList. (Note that you need to set AutoPostBack=true for radioButtonList as mentioned by drs9222.