According to this question on printrbot, the fan is powered by a IRLML2060TRPBF MOSFET (Q3 on the schematic). Specs on digikey say the maximum power from that is 1.25W.


Looking at various 40mm fans, most draw about 100mA @ 12V, so the output of the FET is just barely enough to run them, so running two is out of the question.