Did you enable both ppp connections in load balancer and configure failover?
At this point you can try downloading large files and see if your total d/l speed will be close to the sum of each connection speeds. LB on ZS will kick in only when multiple simultaneous connections are being made.
For me it does not work every time. Sometimes several d/l segments have to be started, other times files have to reside on the different servers. The only technology that always gets LB is torrent with many simultaneous connections.