Batteries in series with different amp-hour ratings.

It is bad practice to connect batteries in series when they don't have the same capacity. The battery with the smaller capacity will be empty before the larger one, resulting in a lower voltage for the smaller battery. At that point things will start to get interesting as the larger battery will start to charge the smaller one through the connected circuit and with reversed voltage. The cell is not designed for being reversed and charged and bad things may happen a.o.: leaking acid and exploding. Neither of these situations are desirable. This is also the reason why most manuals of battery operated devices urge to replace all batteries at the same time.

When in series, the smaller battery will deplete before the bigger one. When a battery reaches its last bits of energy stored, its voltage drops rapidly.

Therefore, after using up more than 1.5Ah, the first battery's voltage will drop significantly, while the other will keep supplying current at 12V, since it still has 8.5Ah left.

You can't let that smaller battery operate in these conditions. This means that you would effectively have a 24V 1.5Ah battery.