Hi Peter,
What protocol(s) do you want to do this for? For HTTP you can clone requests to a second pool. Requests are copied and responses from the second pool are dropped.
HTTP Request Cloning via iRules
http://devcentral.f5.com/Tutorials/TechTips/tabid/63/articleType/ArticleView/articleId/1088513/HTTP-Request-Cloning-via-iRules-Part-1.aspx
It should be possible to do something similar for generic TCP and/or UDP traffic using high speed logging.
If the first server is down, you could send all requests to the second pool. And generally BIG-IP's are deployed in an HA pair, so redundancy at that level is taken care of.
Aaron