In some notebook computer batteries, there are more cells than needed to generate the Voltage the computer needs. So, for example, if you had a computer that needed 9.6 Volts, you could achieve this with either 8 standard 1.2 V batteries, or with 16 1.2 V batteries (two 9.6 V packs in parallel.)

Putting batteries in series increases their Voltage, but does not increase the mAh rating of the pack. Putting batteries in parallel keeps the Voltage the same, but does increase the mAh rating. So if you were to create a double-cell pack, you double the mAh rating. That is how many PC laptops make 'extended life' batteries, they just double, triple, or quadruple the internal number of cells, keeping the Voltage the same, but increasing the mAh rating. (I had a Sony whose 'standard' battery fit totally internally, but the 'double' size pack stuck out the bottom, acting like a rear stand.) Apple has never had battery packs that could reasonably stick out of the computer, so it hasn't been possible to make these double-size battery packs for Apple systems. (Although it might be possible to rig up such a system for the original iBook.....)