Except thermo is from one source and low light is from another source both of which are super imposed on one monitor and do no work in concert with one another, but thanks for trying
Then how would you run it? Cus that's where this whole convo started... do you get to count both at the same time, or do you have to choose?
Random Joe comes home late, and trudges up the stairs to his apartment. He opens the door, steps just inside, and closes the door behind him. Oddly enough, none of the lights that usually turn themselves on for him seem to be working, and the room is pitch black without a hint of light.
Joe has his Oak-Lee's sunglasses with Low-Light and Thermo installed.
Now as I said before, thematically the Low-Light is using light amplification to help you see. If there is ZERO light in the room then they are useless. However, game mechanics say that they will automatically improve Dim or Partial all the way up to Full, thus negating the penalties.
Thematically, Thermo sees using infrared, which doesn't require any amount of ambient light. So that will still function in the utter blackness that is his apartment. However, game mechanics say that it just moves the penalty up 1 category on the chart.
So... thematically, the Thermo is the only thing that would work in this situation, and it would still leave a pretty decent penalty. Moving up from Total Darkness (-6) to Dim Light (-3). Joe can hardly make out that there are shapes in the room. The Low-Light is useless since even after activating Thermo, there still isn't any ambient light for Low-Light to amplify.
However, game mechanics-wise, if you allow them to stack the 2x vision enhancements, Thermo moves the Light effect penalty up one category, from Total Darkness to Dim Light, at which point Low-Light's game mechanic says you automatically move from Dim Light clear up to Full Light. Now Joe can see as clear as day that there's a surprise party waiting to spring.
Now as the GM sitting across from Joe's player... do you let him stack the Thermo and Low-Light, completely ruining the point of using Total Darkness in the scene. Or do you adhere to the thematic fluff and say that he's still got a -3 penalty?