You can add real links in your menu, so you could specify a link to the other app between "a" tags.
see http://docs.splunk.com/Documentation/Splunk/4.3.2/Developer/Step6BuildNavigation
examples of links to a dashboard in another app :
a href="http://myserver.mydomain:8000/en-US/app/webintelligence/business_visitor_trends"
a href="/app/webintelligence/business_visitor_trends"
I want to create a drop down menu in default.xml which has links to other apps. How do i do this using this.