> 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.

I've found the opposite is true. If you are extremely competent at your job your manager is motivated to keep you in place because that's what you're good at. It's easier to promote lower-accomplishing people-pleasers because it won't affect _their_ software throughput as much (higher level engineers write less code).