I know someone who did this for a over a decade.

Even got promoted to management in one of the two jobs! They would sometimes have to take calls from both jobs with one earbud in each ear connected to each laptop.

The industry the companies were in are not known for employing the best and brightest programmers... so I can understand being able to output enough to keep everyone happy. But it still boggles my mind that they never slipped up and got caught.