1. Most dev shops work in a similar way:
During my career I have worked as a middle manager at multiple companies where I was on the other side of the table. That is, I outsourced projects to dev shops. All of them would bring a technical lead to the initial interview, and all of them would pass the work to one or more junior devs. Of course, they'd say that the tech lead was overseeing the work, but in some cases it was only marginally true.
I came to expect that this bait-and-switch is how larger oursourcing shops work, so later I tended to hire individual contractors where it wasn't a problem. However, since that meant more of my time was spent overseeing the contractors, this approach didn't always sit well with my own boss who wanted higher volume output. So often I was forced to play this game with outsourcing dev shops.
Are those dev shops being unethical? If they got the job done, and they told the client that the junior employees handled the work and the tech lead simply approved the final submission, then I think most employers would be ok with it.
2. Working 2 full-time jobs is ethical IF the employers know that's what you're doing:
If an employee is so proficient that they can do their work at two different full-time jobs, then most employers shouldn't have much reason to complain. But generally we're talking about lying to the employer and since employment contract almost always include an exclusivity clause then it's no minor verbal lie. There's no way this is more ethical than what the dev shops do.