I’m curious how seniority can be the fundamental for this. As you say, you have to be good enough to finish a full-time work less than 8 hours, that means you are working for a junior position? You can only do that if you work at a lower-level position, right? I dont think being senior means you can finish your work day sooner than juniors do.
If I get if right, you need to be good, then find 2 jobs at the level lower (even far lower) than yours. However, I’m not sure companies would need a full-time senior for a junior position.
Another question: you do 2 jobs simultaneously (9h-17h for the both companies: coding for company A while meeting in company B) or consecutively (9h-17h for company A then 17h-xxh for company B).