Over-volting is not completely out of the question, but I wouldn't bet on it. If I had to take a wild guess, it would be to provide additional current to drive the built-in flash that fast? Does Nikon have any cameras with no built-in flash nor fixed grip (a la 5D)? Seems sort of silly, though. It's always possible it's just a marketing ploy.
Perhaps overvolting could
be out of the question since electronics are very sensitive to over and undervolting. While over "current" on the other hand could be a very possible reason. Power supplies rely on capacitors and regulators to steady the voltage supplied to the curcuits.
Under higher FPS, the regulators will drain the battery much faster as they demand more current to make up for lost voltage to charge the capacitors. With 2 batteries there, the risk is lowered, in this configuration they are run in paralell, i.e. 2x current, not in series , i.e. 2x voltage. Then again as Neuro says, it is a carryover form film days and they want to sell you a grip and batteries.