Question

Why did you use two coffee cups during the experiment instead of just one cup?

in case a hole was punctured in the inside cup

to add more weight in order to stabilize the calorimeter

to hold all of the water in the experiment

so that the water temperature would not change at all during the experiment

to prevent heat from escaping from the calorimeter

Answer 1

To prevent heat from escaping from the calorimeter.

Two nested cups make a simple insulated calorimeter: the extra cup and the air gap between them reduce heat loss by conduction and convection, so the system is closer to adiabatic and your temperature change measurement is more accurate.