iRule for cloned request and response traffic

I'm afraid it doesn't work that way. I use this analogy a lot, but you almost have to think of an event like a "bucket". The bucket gets filled with an aggregate of commands and values, and then gets dumped out and readied for the next request. So in the following example:

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    set orig_uri [HTTP::uri]

    HTTP::uri "/foo"
    clone pool clone_pool

    HTTP::uri $orig_uri
    pool $default_pool
}

It does indeed send traffic to both pools, but the resulting URI for both is the URI set in the last HTTP::uri command. Your best may be to perform a sideband call with the modified data. It'll eat up a bit more CPU, but you'll have much more flexibility.