Thermodynamically speaking, a radiator is just another type of heat exchanger where heat from one medium and transfer it to another. Best case scenario is when the input air is coldest, then the exit liquid is at its lowest. But stacking up the radiators forces each additional radiator to receive the heated exhaust from the one before it. So the liquid won't cool as much, but you'll have to do your own experiments to be sure if the difference is worth the space savings of stacking them.
If you stack too many radiators, the last set of radiators isn't radiating any heat from the liquid since the heated air coming in to probably too close to the liquid temperature in. Once the temperature of both air and liquid inputs are the same, they exist at the same temps, there is no heat transfer except for whatever the radiator absorbs such that it feels hot.