Background:  For a test in QUV we can choose between two conditions that give the same amount of total energy: 
Test A: Irradiance 0.65W/m2 for a time of 3000h.
Test B: Irradiance 0.83W/m2 for 2500h.

The testing samples are films, with a thickness between 80 and 100 micron.  At the end of the each test, we verify the change of color in grey scale and the mechanical property retain.

Can we compare the two results?

Answer: It is common to assume that the two exposures are the same, but in many cases the two would yield different results. There are complex synergies between irradiance, heat, and moisture. Generally, when this question is studied, there is a much bigger difference between irradiance setpoints. The question is something like this: if you double the irradiance of the exposure, can you cut the test time in half and achieve the same results?

Sometimes there is a linear relationship between irradiance and speed of degradation, but sometimes not. In the example in the question, the difference is small, so there is greater likelihood that the results are comparable.

Still, I would conduct your own study and find out. I would be interested in learning of your results.