Regarding nested if irule.

i used switch here because i think it is a bit easier to read.

 configuration

[root@ve11b:Active:In Sync] config  tmsh list ltm rule qux
ltm rule qux {
    when HTTP_REQUEST {
  log local0. "uri = [HTTP::uri]"

  switch [string tolower [HTTP::path]] {
    "/test.html" {
      switch [string tolower [URI::query [HTTP::uri] username]] {
        "testuser" {
           do something
          log local0. "/test.html with username=testuser"
        }
        "testuser1" {
           do something
          log local0. "/test.html with username=testuser1"
        }
        default {
           do something
          log local0. "/test.html without username or with username but testuser/testuser1"
        }
      }
    }
    "/test1.html" {
       do something
      log local0. "/test1.html"
    }
    default {
       do something
      log local0. "default"
    }
  }
}
}

 /var/log/ltm

Feb 28 22:32:12 ve11b info tmm[14159]: Rule /Common/qux : uri = /test.html?username=testuser
Feb 28 22:32:12 ve11b info tmm[14159]: Rule /Common/qux : /test.html with username=testuser

Feb 28 22:32:17 ve11b info tmm[14159]: Rule /Common/qux : uri = /test.html?username=testuser1
Feb 28 22:32:17 ve11b info tmm[14159]: Rule /Common/qux : /test.html with username=testuser1

Feb 28 22:32:25 ve11b info tmm1[14159]: Rule /Common/qux : uri = /test.html?username=somebody
Feb 28 22:32:25 ve11b info tmm1[14159]: Rule /Common/qux : /test.html without username or with username but testuser/testuser1

Feb 28 22:32:48 ve11b info tmm[14159]: Rule /Common/qux : uri = /test.html?surname=something
Feb 28 22:32:48 ve11b info tmm[14159]: Rule /Common/qux : /test.html without username or with username but testuser/testuser1

Feb 28 22:32:56 ve11b info tmm1[14159]: Rule /Common/qux : uri = /test1.html
Feb 28 22:32:56 ve11b info tmm1[14159]: Rule /Common/qux : /test1.html

Feb 28 22:33:04 ve11b info tmm1[14159]: Rule /Common/qux : uri = /abc.html
Feb 28 22:33:04 ve11b info tmm1[14159]: Rule /Common/qux : default