This can only work at extremely dysfunctional companies with incompetent management. I suppose there's an opportunity here for some unethical people to get some money from those companies until they collapse. In any reasonably well-run company these people would be fired quickly.

Gordon Gecko might say these people are performing service to the public by helping poorly managed companies go bankrupt faster, but it's still unethical because the goal seems to be to massively underwork at each job.

On the other hand, I think that it should be reasonable for software engineers to work two jobs, as long as they are not competitors and there are no intellectual property concerns. Just like any other employee, some people want to work two jobs, and I'm actually disappointed it's not more common to arrange for multiple legitimate jobs (two part time, or one full time one part time) in the tech industry.