Passive FTP session load balancing from the same client

I don't think you are going to be able to load balance the data connections. Unless you have some magic FTP server the data connection must go back to the same server that the control connection is on.

When doing passive FTP the server opens a port to do the data transfer on and then tells the client what port it opened. The client the connects to that port. So if the control connection is to server 1 it will open the port for the data transfer, if you try to send the inbound request from the client to server 2, server 2 will not know anything about that port.