1) there is a constant amount of overhead that is always needed regardless of how long you work for in a week. Meetings and collaboration to figure out what to build, discuss issues etc. if these take up 10 hours a week a full time dev has 30 hours to do other things where as a half time dev only has 10 hours. So 2 half time dev just doesn’t get as much done as 1 full time. I guess this is also why 2 engineers don’t get twice as much done as 1, all things being equal.
2) Most part time dev candidates I’ve seen aren’t doing just a single part time job, they are doing either multiple part time jobs or a full time job + part time job. Either way the mental energy is already gone.
Maybe there is a way to make part time work, I just haven’t figured it out yet.