Generally speaking, in most states in the US, it would not be permissible to prohibit moonlighting for an unrelated second employer in an employment contract.

Generally speaking, in most states in the US, it would not be permissible to prohibit moonlighting for an unrelated second employer in an employment contract. There are exceptions, but in general, this is the case. It is, however, also the case that employers can require you to inform them in advance of any moonlighting, they can demand an opportunity to vet the moonlighting to verify that it is legitimately unrelated and that you are not being hired by the second employer due to a desire to gain either and trade secrets or any favors relating to the first employer (or any other similar potential conflict of interest), and they can require that you deliver the same workload for them that you would in the event that you did not do any moonlighting. Of course, actually going through the whole officially-saying-you-are-moonlighting process is tedious, annoying, and can lead to conflicts with the first employer (especially since they'll start looking closely at your performance, which they might not otherwise do), so many people who have agreed to an agreement like this moonlight without telling the first employer -- and then they are subject to disciplinary action if they get found out. (Also lots of people accidentally or carelessly end up using some first-employer asset/resource when moonlighting -- especially in a remote-work world where your employer might have outfitted your home office for you with property that you have agreed that they own even though everybody knows that if you left the employer they wouldn't ask for it back)