Most of my salaried positions have been "at will" with no employment contract in place, so the only thing that corporate policy tells me in the employee manual

Most of my salaried positions have been "at will" with no employment contract in place, so the only thing that corporate policy tells me in the employee manual is that I may be fired if they find out. But as an "at will", I could also be fired for any reason, or no reason at all. As an "at will" employee, the one and only consequence I have to fear is being fired. There is no contract to breach. And in that situation, I would feel a lot more comfortable about going from two jobs to one than from one to zero. The incentive is overwhelmingly in favor of doing it and hiding the evidence, rather than not doing it at all. If you do everything that is expected at both jobs, neither has any reason to investigate.