Hello sporkman,
I thought you had been using the Max series. On the Balance series you’ll need to click on the Network tab. This should take you to the WAN info. Click on the WAN name and in the first Connection Settings box you can find the Connection Type. By default this is set to Always-on. Change this to Backup Priority. A drop down will appear next to the Backup Priority.
If I’m understanding you correctly, once a session is created the traffic will continue to use that session. So once a request is sent out, the response for that webpage will come back using the same WAN that it was sent out on.
I’m unsure if this may help but we do have a description that was put together by Alex of how our Load Balancing Algorithms work.
I hope this helps!