The Weird and The Wonderful

<h2>First Header</h2>
  <div>Item 1</div>
  <div>Item 2</div>
<h2>Second Header</h2>
  <div>Item 3</div>
  <div>Item 4</div>
  <div>Item 5</div>

<script runat="server">
  Private Sub myRepeater_PreRender(sender As Object, e As System.EventArgs) Handles myRepeater.PreRender
    For Each item In myRepeater.Items
      If SomeCondition1() Then
        Dim headerContainer As PlaceHolder = item.FindControl("myHeader")
        AddHeader(headerContainer, "First Header")
      End If
      If SomeCondition2() Then
        Dim headerContainer As PlaceHolder = item.FindControl("myHeader")
        AddHeader(headerContainer, "Second Header")
      End If
    Next
  End Sub
</script>

<asp:Repeater runat="server" ID="myRepeater">
  <ItemTemplate>
    <asp:PlaceHolder ID="myHeader" runat="server" />
    <%-- ...Rest of item template... -->
  </ItemTemplate>
</asp:Repeater>

<asp:Repeater runat="server" ID="outerRepeater">
  <ItemTemplate>
    <h2><%# HttpUtility.HtmlEncode(DirectCast(Container.DataItem, ItemCollection).HeaderText) %></h2>
    <asp:Repeater runat="server" DataSource="<%# DirectCast(Container.DataItem, ItemCollection).Items %>">
      <ItemTemplate>
        <%-- ...Rest of item template... -->
      </ItemTemplate>
    </asp:Repeater>
  </ItemTemplate>
</asp:Repeater>