Dying models question.

Hello! I am concerned about dealing with dying models in a situation where an attacker has multiple attacks. Lets say a model with 2 attacks allocates both of those attacks onto a single model. The target fails their first save and is knocked over as a fallen model. As the second attack roll has yet to be resolved and was allocated to the now fallen target, can be it be used to coup de grâce the target. Or is that second attack wasted and another model must use an attack to finish off the fallen enemy? This question has become relevant in my battles involving necromancers or a large amount of models with first aid. Thanks in advance for any answers.