Two Pools assignment to single VIP

You'd generally use an iRule here. For example:

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    if { [active_members $default_pool] < 1 } {
        pool backup_pool
    } else {
        pool $default_pool
    }
}

That's a great point, Farshadd. And indeed both options are viable. The iRule presents a logical approach to load balancing based on the availability of specific pools, while priority group activation puts all of the servers in the same pool and assigns a higher priority to some. In this case, the lower priority servers are only used if the higher priority servers are unavailable.

I think you should always use the provided configurations when possible. Unless there is some additional logic or functionality, I would use Priority Group Activation here. iRules add additional complexity, reduce efficiency, and in my experience, the folks supporting the applications and/or devices struggle more with iRules. Less iRules = less late night calls with questions from the NOC when something goes wrong.