By default, you can't; it would be a security loophole.
But you can set it up depending on your server; for
Tomcat, you need to first set the attribute 'crossContext' equal to true for the webapp's <Context> element.
You can then get the other context with
ServletContext.getContext("/otherWebApp"). On this, you can then use the usual getRequestDispatcher() and RequestDispatcher.forward() methods.