Is it possible to monitor for two expressions?

I think this is totally possible - you'd want to craft a regular expression that contains both content that will be on the page under normal conditions, and a string that appears when a 404 is displayed. Take a peek at the 2nd post in this thread for an example:

 

 

http://devcentral.f5.com/Default.aspx?tabid=53&forumid=32&tpage=1&view=topic&postid=3213232132