URL Rewrite and LB at HTTP request

The easiest way is to create two pools, one for your www1.domain.com and one for your www2.domain.com. I'm assuming from your post that you have both domains on the same virtual and with that assumption, you can create an iRule that switches on the domain and then dynamically assigns which pool to use.

when HTTP_REQUEST {
  switch [string tolower[HTTP::host]] {
    "www1.domain.com" {
      pool www1_pool
    }
    "www2.domain.com" {
      pool www2_pool
    }
  }
}

where www1_pool and www2_pool are the pools containing the app servers.

-Joe