Not a silly question at all.
Think of twin packs as twin fuel tanks. Both have in internal fuel pump so pressure isn't an issue.
In a parallel system, both tanks have been connected to the motor by the ESC. Think of it as a throttle body in a fuel injected engine. Since both tanks have the same pressure going to the throttle body, they both push the same amount of fuel and any excess goes back to the appropriate tank. The effect is that you double your run time while keeping your pressure(AKA voltage) roughly the same through out the run.
In a series system, this all changes. You have one tank force feeding the second. The fuel then goes to the throttle at double the pressure, with all excess fuel returning to the first tank. The effect is like installing fuel injectors with larger nozzles, more fuel goes to the motor so you have more power and only one tank worth of run time. The issue with running packs in series is the higher voltage increases the heat generated and wears out ESCs and motors faster.
Hope this all makes sense