Your questions hit on the vital points of batteries. As you suggest, two 6 Volt batteries wired in series can start a car just as well as one 12 Volt battery. Whether or not they can supply more or less amperage depends several details: the resistance of the wiring connections, the internal resistance of the batteries, storage capacity, etc.
The lantern batteries you mention, however, are unlikely to be able to start a car. The reason is that they just can't supply enough current, even in the short run. Formally, you can say that they have too much internal resistance, which adds to the resistance of the load, and thus lowers the amount of current for a given driving voltage. More physically, the surface area of the electrodes is lower and the rate at which the ions move around is also, I believe, lower than in a lead-acid battery. As a result, the net number of ions arriving per time at an electrode (the current) is lower.
In addition, of course, there's the question of whether power can be supplied for long enough to get the engine going. The battery's energy storage depends not only on how much power it can supply but also on how long it lasts. The issue of total energy storage boils down to two main points: the type of battery, for example, lead acid, alkaline, NiCad, Lithium, etc, and the volume of the battery itself. The more a battery weighs, the more volume, the more storage capacity. A number that can characterize this is the number of Volt-Amperes a battery can deliver. An equivalent number is Watt-Hours. For example common alkaline batteries that run flashlights and small electronics equipment come in various sizes. Your flashlight probably uses D-Cells, a TV tuner probably uses AA-Cells, etc. They supply the same Voltage, 1.5 V, but they have different weights. In general for a given battery type, the heavier the battery, the more total power it can supply,
The Wikipedia article http://en.wikipedia.org/wiki/Battery_%28electricity%29
is pretty good at explaining all this.
By the way, hooking a 6 Volt bulb up to a 9 Volt battery is not good for either the bulb or the battery.
LeeH and Mike W.
(published on 10/12/10)