Hi Icube,
I think you're thinking about this as if its a router.Simply put It doesn't care about paths. All it cares about is outgoing interfaces and sessions. If a new/different session is created ( new session being one with different IP, Source Port, Destination Port, ....etc. ) it will chose a different outgoing port for it. The SRX is a statefull firewall so it relies on the session table for loadbalancing.
