Multiple sites one server health check

You can attached multiple HTTP/S monitors to the one pool if that's what you want to do, but that will mark a server down for all sites with that setup. You may want to consider using multiple pools each with the unique HTTP/S monitor for each site. The you can use an iRule that will do L7 load balancing for each host header, that way each site can be monitored independently when behind one VIP.

You could create each pool with the name of the host header(www.yoursite.com_pool) and use a iRule like this. You would still need a default pool for your site just in-case you don't have a pool for an unknown host hear if you want to handle that traffic with a catch all site.

when HTTP_REQUEST {
    pool "[string tolower [HTTP::host]]_pool"
}