I am not sure why this is not working.
<asp:Label ID="lblTitle1" runat="server" Text='<%= string.Format("{0} <br /> {1}","Part 1", "Part 2") %>' ></asp:Label>
I just want to combine two resource string as text of label?
Use databinding instead, Text='<%# String.Format(...) %>'
and make sure you call DataBind() somewhere in your code.