How to build a light sensor for PWM dimmed light (for comparison against CC dimmed light)?
I'd suggest that the Intersil ISL29125 might be suitable for your application. Available on a breakout board from either Digikey or Sparkfun for under $8.

These are I2C devices with very high resolution and integration times that are adjustable to reject 50/60 Hz and above modulation effects, so they should reject your PWM frequency very well.

You could calibrate 2 individual sensors or use just a single one in a strict comparison mode. 

There are multiple solid state light sensors available, but this is the only one I've seen with such good calibration control.

Depending on how professional a solution you are developing there are Arduino libraries for this device from Sparkfun, though they lack much in the way of calibration capability.