How big are these 12 V fans? The -12V rail on a computer PSU has a very low current capacity, generally less than 1 amp so there really isn't much power available to work with.
More complicated but more reliable way to do it would be to use something called a "Voltage Doubler", it uses 2 capacitors and 2 diodes to approximately double the voltage on its output, you would need to make sure you use components of the appropriate rating in it which will depend on the current draw of the fans.
Somehow manager to forget all my power electronics in like three months sorry. That voltage doubler is for an AC in and provides a DC out, which explains why when i first wrote that i was trying to figure out why it didn't need a boost converter to increase the voltage. That won't actually work since it is a DC input signal sorry.
It isn't exactly the safest one if you are going to have it in an exposed area due to how the connectors are, but it will get the job done. I would put it in a secured area where things won't fall on it but it will have some room to breath.