This has been done many times, it's often called "buddy pairs." That's putting pairs of batteries together in parallel, and then putting these buddy pairs in series. Some racers have even done triplets.
I bought a 72 volt system with six deep cycle batteries. At 30 miles per day per day round trip it looks like I will need more amp hours to use this vehicle.
I was planning to add to the system. Six batteries in series and six in parallel to extend the range. Would this be ok, or will I need to change the motor and controller?
If you know a link where this issue has been addressed, please let me know.