First of all, test your hypothesis that both the yellow and brown wires being 12V+ sources. Both are probably wires for running the larger fan in low and high speeds with the fan speed controlled by the computer. As for the smaller fan, that would likely be used for whenever the AC is turned on. With regard to operating both fans separately, that can be done using relays. The triggers for the relays would be an engine temp sensor and a tap into the AC compressor clutch.
Hope this helps...Merry Christmas!
Hope this helps...Merry Christmas!