If I may add, it also depends on how dark you (and your eyesight) are willing to go. BC35 + CS35 is still drivable. In reality, it's probably going to be a tad lighter than just a layer of CS20. I wouldn't recommend that you add a layer of CS20, though, as that would be in super dark territory.

I had a friend before, on his galant Sigma he had 2 layers of BC35. Yes, BC35. The result from the outside was that you couldn't be seen. Not sure if there was any glare at night. Personally, I wouldn't do it, coz one thing that BC has that CS has perhaps less of is glare.

I haven't tried 2 layers on my windshield yet. I had Panther 5 on my 626's windshield a long time ago as the darkest that I've ever used for more than 6 months. IMO, i'd have the BC35 stripped off and BC20 installed.