Certainly would agree with running 2 of the 3 S's in series as mentioned above to save the craft if you drop a cell. Having an extra battery in parallel isn't going to double flight time anyway. At most it's a 40% to 50% reserve to get the get the rig back on the ground safely if you stay on...
Being the owner of a flat Jib 8 I wonder about ESC placement that would allow for that. My ESC's are sandwiched between the mounting plates, between the booms for protection