assigning two pools to one virtual server

Can you provide more info on why you want to do that? Do you have any requirement depending on the requested URLs/URIs ??

I have a customer that has a requirement to have two separate pools on one virtual server. The first pool is primary and the second will be passive/standby. This is due to the application he is running.

If your customer intention is to have servers in idle state and take over when primary servers are down,

you can have one pool with all members and enable priority group activation. This way lower priority group will not be used until your primary(high priority) servers goes down.

Priority Group activation is best method but still customer is not happy go with iRule selection. if primary pool fail traffic will move to secondary pool.

    when HTTP_REQUEST { 
     if { [active_members [LB::server pool_1]] < 1 } { 
          If the default pool is down, redirect to other pool
            pool alternate_pool_2
        }    
    }