TS just needs to find low amperage fans so as not to overstress the USB port...

ive done this mod before.. connected two 12cm fans to a single usb cable/port.

i had this mod connected to the USB port of my office's NAS....and it has been running 24/7 for the past 3-4 years with no issues watsoever....only stopped to clean off dusts every now and then...last cleaning oso was about 2 years ago.....still running til today.

but ull definitely need a multimeter to figure out which wire is which. coz inside the USB cable is 4 seperate cables of different colors.. and not all USB cables has the same colors.

and of course... the 12v fans will definitely run slower since the USB only supplies 5v.