You can't do it with 2 clocks AFAIK (the clock always shows the time in the local timezone by default), but you can add extra locations in the drop-down menu of the clock applet so that seeing UTC is one click away (this is not yet possible with indicator-datetime though, although I think it's planned).
The GNOME clock applet, can't display two times on the panel, but it can show two times in the drop down.
Right click on the clock and select "Preferences." Goto the "Locations" tab and press "add."
Then, select "Greenwich Mean Time" from the "Timezone" list.
You can't do it with 2 clocks AFAIK (the clock always shows the time in the local timezone by default), but you can add extra locations in the drop-down menu of the clock applet so that seeing UTC is one click away (this is not yet possible with indicator-datetime though, although I think it's planned).
Try to install indicator-datetime
indicator-datetime will appear in Indicator applet in your tray icon, this applet for your local time,
and the old one just change the configuration in /apps/clock_applet/prefs/gmt_time with gconf-editor, for yuur UTC/GMT time.