Using two or three wax cubes at once may give you a longer-lasting scent than using one, but it does not guarantee double or triple the fragrance time. Extra wax contains more fragrance overall, yet how quickly that fragrance reaches the air depends on the wax formula, your warmer's temperature, and how the wax melts in the dish. You may notice a stronger scent, a longer-lasting scent, or little difference.
If one cube already gives you enough fragrance, try using your cubes one at a time and replacing the wax when the scent fades. That lets you save fresh wax for later instead of warming it all at once. Use extra cubes when one is not giving you the scent level you want, staying within your warmer's recommended capacity. More cubes can change the experience, but they are not a reliable shortcut to two or three times the scent life.
If you want to compare portions, test one cube and two cubes on separate occasions using the same fragrance, warmer, and room. Keep track of actual warming time and when the scent drops below the level you enjoy. Starting with fresh wax each time gives you a clearer comparison than adding a fresh cube to one that is already spent.
Two cubes may give you more time before changing the dish, but that is a different question from getting the most total use out of a pack. If a single cube works well for you, saving the second for later remains a sensible option.