iRule: Same host two ports, if port 1 is not listening traffic should not go on port 2

You could use a Local Traffic Profile or an iRule to check that both pools are available before selecting the pool for your FTP service.

I've provided an example of an iRule below:

when CLIENT_ACCEPTED {
    if {([active_members pool1] < 1) && ([active_members pool2] < 1)} {
        pool pool2
    }
}