Effect of 'drop/discard' in HTTP_RESPONSE event?

Since your second server is connecting to the BIG-IP, it's really more of a client, and there's no automatic way to hook two clients together like that. The only solution I can see is to do something like:

 This is on your main VIP that your clients talk to
when HTTP_RESPONSE {
  set resp ""
  while { $resp == "" } {
    after 100
    set resp [table lookup $clientkey]
  }
  HTTP::respond $resp
}

 This is on a different VIP that your "second servers" send their "requests" to
when HTTP_REQUEST {
  set clientkey [HTTP::header X-ClientKey]
  table set $clientkey "HTTP/1.0 200 OK\r\n\r\n[HTTP::header X-Client-Data]"
  HTTP::respond "HTTP/1.0 200 OK\r\n\r\n"
}

This is just for illustrative/explanatory purposes; I haven't checked this code at all. It's not very efficient, and of questionable scalability, but likely the best you can do. You should test it out to see if it can meet your capacity needs. Hope this helps!