You can't make 10 V from two times 5 V unless they are completely separated. In that case you can place them in series.
The best way to get 12 V from 5 V is to use a switcher, or SMPS (Switch-Mode Power Supply). Buck types deliver a lower output voltage than the input, boost give a higher output voltage, so you want the latter. 5 V in to 12 V out is a piece of cake for a boost switcher.
Switchers can be very efficient (little power lost), and therefore often very compact as well.
Keep in mind that you can't fool the laws of thermodynamics: if your fan would need 200 mA, for instance, that's 2.4 W, then the 5 V supply has to deliver that too, and 2.4 W at 5 V is 480 mA, not 200. Make that probably 600 mA because there's still some losses in the switcher.
So check how much current (or power) the fan needs at 12 V, and that the 5 V supply can deliver the required power.
If you want to power a 1.2 W fan from your USB port you'll need more than 240 mA from it, probably around 300 mA. A USB port can deliver up to 500 mA, but that has to be negotiated with the host (part of the USB software protocol). Without negotiation you can only get 100 mA, and that's not enough.
Powering electronics from the USB port, TI application note