How do I use an asp.net user control in another user control?

<%@ Register TagPrefix="ssctrl" TagName="photo" Src="controls/photo.ascx" %>

<ssctrl:photo ID="Photo" Key="<%# Eval("PageTemplatePK") %>" runat="server" />

<ssctrl:photo ID="Photo" Key='<%# Eval("PageTemplatePK") %>' runat="server" />