Not entirely true. if you had the option of a thick vs stacked thin rads the performance can come close to equal but this is stacking two thick rads to make the most of the space.
From what I've seen you're better off not stacking radiators, ever. The hot air from the first radiator makes the second radiator extremely inefficient at shedding heat.
If you had the option to keep them separate you could but staking them isn't going to make your performance worse it just wont scale perfectly.