Somewhat depends on the circuitry. In general, if you increase voltage to a dc motor you increase the speed (note caveat below) and vice versa. But the circuit could have some sort of voltage regulation in it to prevent that, though not likely given that it was designed for a very specific oem battery. I don't know if the aftermarket 9v adapters have any voltage regulation.

Battery voltage declines with usage. Putting 2 9v batteries in parallel still gives you 9v, but over a longer period of usage. So IF your wind speed lengthens as the battery wears out, then 2 in parallel will delay that (as ambaker said).

You could put them in series (so 18v) which in most cases will increase the speed of a motor (if no voltage regulation). It may also fry the motor if it's not designed to handle the extra voltage. But you'll likely get at least one fast 12 exp roll through it.