Assuming you only work 20 of your 40 hrs/week in each job, you are still working a fulltime job with 2x your salary. 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.
Now work a couple of years in this job, and soon you'll be able to complete its responsibilities in 20ish hrs/week. Every job gets easier overtime in that sense.