Direct a % Traffic to Two App Pools

You could do this with a counter.

when RULE_INIT {
   initialize the global counter
  set ::request_num 0
}
when HTTP_REQUEST {
   Increment the counter
  incr ::request_num
  if { $::request_num < 10 } {
     request 1-9 will go to poolA
    pool poolA
  } else {
     the 10th request will go to poolB
    pool poolB
     Reset the counter
    set ::request_num 0
  }
}

Basically this will keep a request counter. For the requests 1-9 it will use poolA (90%). For the 10th request (the other 10%) it will use poolB and then reset the counter.

I'm sure there are other ways to do this but this should get you started.

-Joe