Hi Rableen,
When your Portal users are going to backend it will be connecting with the portal system object of the backend server and there you will the object type will be dedicated connection which will be the host given of one app server only where you are facing the user ;load. Please create a new system object with type load balancing and give MS hostname and details and you will see your portal users are being routed to different app servers.
Regards
Naveen Garg