"If someone is competent enough to sustain 2 jobs, they will also get quickly promoted to 2x their salary if they are working one job."

Between the two extremes (mediocre and rockstar/10x), there are many 'competent' people who can't get promoted to their 2x salary jobs. And these people are competent in a particular niche, not like those who aced programming olympiads, compiler gurus, kernel hackers, etc.