Not allowed on domestic awards.
"One stopover en route to your final destination or one open jaw is permitted for a flight award on United between the continental U.S. and Hawaii or between the U.S. and the Caribbean, Asia, Australia, New Zealand, Central and South America or Europe. No stopovers are permitted within a geographic region."
http://www.ual.com/page/article/0,1360,1141,00.html