That got my attention as well. Section 19 stipulates the basics for cars that require cells, and even there, it says nothing about location other than the 12" rule.

The ITCS also says nothing about front allowances for mid engine cars.....but allows you to replace[b] the stock tank with a cell within 12". IF that new location encroaches the driver compartment, you must construct a metal bulkhead, and the cell must be 6" above ground unless it's located within bodywork or the original floorpan.

I see nowhere that the rule allows stock AND new cells to co-exist, and no any rule that allows you to locate the cell more than 12" from the stock tank (perimeter to perimeter), except the notes on the spec line for certain BMWs.