pool selection

I think the iRule below fill meet your requirements.

when CLIENT_ACCEPTED {
    set pri_pool "/Common/pool_a"
    set sec_pool "/Common/pool_b"

    if {[table lookup active_pool] ne ""} {
        if { [active_members [table lookup active_pool]] >= 2 } {
        }
        elseif { $active_pool eq $pri_pool } {
            table replace active_pool $sec_pool indefinite
        }
        else {
            table replace active_pool $pri_pool indefinite
        }
    }
    else {
        table set active_pool $pri_pool indefinite
    }
    
    pool [table lookup active_pool]
}

Have fun,

     --Niels

_Dmi_ You can do this with priority group activation in 1 pool rather than having 2 different pools.

https://my.f5.com/manage/s/article/K13525153

Hii

You can try this

when HTTP_REQUEST {

set pool_a_members [LB::pool pool_a members]
if { [llength $pool_a_members] > 0 } {
pool pool_a
} else {
pool pool_b
}
}

or check this post similar

https://community.f5.com/t5/technical-forum/how-to-write-an-irule-to-redirect-traffic-to-different-pool/td-p/84432

Best Regard