Repeater in Repeater

In the parent repeater, attach a method to the OnItemDataBound event and in the method, find the nested repeater and data bind it.

Example (.aspx):

<asp:Repeater ID="ParentRepeater" runat="server" OnItemDataBound="ItemBound">
    <ItemTemplate>
        <!-- Repeated data -->
        <asp:Repeater ID="ChildRepeater" runat="server">
            <ItemTemplate>
                <!-- Nested repeated data -->
            </ItemTemplate>
        </asp:Repeater>
    </ItemTemplate>
</asp:Repeater>

Example (.cs):

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ParentRepeater.DataSource = ...;
        ParentRepeater.DataBind();
    }
}

protected void ItemBound(object sender, RepeaterItemEventArgs args)
{
    if (args.Item.ItemType == ListItemType.Item || args.Item.ItemType == ListItemType.AlternatingItem)
    {
        Repeater childRepeater = (Repeater)args.Item.FindControl("ChildRepeater");
        childRepeater.DataSource = ...;
        childRepeater.DataBind();
    }
}