Just noticed the fan noise level had increased and just looked. The large fan was the only one working. These fans, if working properly, will always run at the same time. If only one fan is running something is wrong. There are different speeds the fans will work at, but if the small fan is out, the large fan will kick in at high speed, by default. The circuitry for the fans is located on the backside of the large fan motor and controls both of the fans.Yes, you could probably source brushes and springs but as you say it's not worth the hassle, especially if they make noise.
How did you see it was not working? Just because one was stopped? Or is it a way to test them like with VCDS?
I don't think the car would overheat with only one fan though but it's probably better from an A/C standpoint if they both run.
Just turn on the AC and within a moment the fans should turn on. I am not sure if there is a VCDS test, but I never received any fault codes nor were there any codes when I plugged into the car.