There's no law that says you can't work two jobs (obviously)
But chances are good that you entered into some form of agreement with your employer that forbids holding another full-time job. You also have NDAs and such agreements that could be argued to be broken if you're working at two companies simultaneously if they have any possible competition with each other.
In practice, a company isn't likely to go after an ex-employee for damages in this case unless there really was a trade secrets violation.
What happens, though, is that the company that realizes the double-work scheme is happening calls up the other company and they both fire the employee. All of the person's managers and coworkers usually hate the person because they were likely sandbagging and pawning work off on coworkers (as is necessary to put in half time at any halfway competent company), so they can't get a decent reference.
Now they're unemployed, without references, and starting over again. Not the end of the world in today's job market, but I expect reference checks for remote work will continue to get much more aggressive to catch situations like this.
What's unlawful around working two jobs? Happens at high end (consulting CTO John Carmack, or Twitter & Square CEO Jack Dorsey, and in the US happens significantly often at the lowest end (retail workers in two jobs).
It's unethical in that the two employers are unaware of that fact, and he's advising deliberately not doing work in bad faith (whereas the boards of Meta, Twitter and Square were aware of the previous situations, and your retail shift manager knows he's not scheduling and paying a living wage), but illegal probably not.
But chances are good that you entered into some form of agreement with your employer that forbids holding another full-time job. You also have NDAs and such agreements that could be argued to be broken if you're working at two companies simultaneously if they have any possible competition with each other.
In practice, a company isn't likely to go after an ex-employee for damages in this case unless there really was a trade secrets violation.
What happens, though, is that the company that realizes the double-work scheme is happening calls up the other company and they both fire the employee. All of the person's managers and coworkers usually hate the person because they were likely sandbagging and pawning work off on coworkers (as is necessary to put in half time at any halfway competent company), so they can't get a decent reference.
Now they're unemployed, without references, and starting over again. Not the end of the world in today's job market, but I expect reference checks for remote work will continue to get much more aggressive to catch situations like this.