Try visiting
http://www.kplug.org/glade_tutorial/...l/gtemp_1.html
It is a link to the Glade tutorial which only took me about 2 hours to complete and it teaches you the answer to do what you wanted and more.
If you only want the answer to your question, here it is:
GtkWidget *about2;
about2 = create_about2 ();
gtk_widget_show (about2);
where you must replace "about2" with the name of your second window.