two whens?

You can have as many event handlers (when statements) in your iRule as you wish.

when HTTP_REQUEST {
   do something
}
when HTTP_RESPONSE {
   do something
}

In fact, you can have mulitple duplicate events in the same iRule (as long as you specify a different priority for them.)

when HTTP_REQUEST priority 100 {
   do something
}
when HTTP_REQUEST priority 200 {
   do something else
}

The default priority for an event handler is 500. Valid values are 0 to 1000 and they are processed from lowest to highest.

-Joe