F5 UAG SharePoint 2010 (NO DIRECT ACCESS)

By default, cookies are host-based, meaning they are tied to the host name. When you click the link of webapp2 your browser sees this as a new connection for which it has no cookies. And no cookie, no persistence. So ultimately you need to be able to persist across host names, irrespective to the fact they're on the same VIP. The built-in cookie persistence profile doesn't have the ability to set a domain cookie, so that isn't an option. Other persistence methods, however, do allow you to "match across". Source address persistence is an obvious option if you control the IP space. Otherwise is there anything in the link to webapp2 that comes from webapp1? Another domain cookie? You may even consider rolling your own cookie persistence to set a value that the browser will faithfully transmit to both hosts.