I did two full-time jobs for a month as part of changing jobs fifteen years ago and it’s exceedingly intense but otherwise was fine; eighteen hour waking days leave a lot of boredom time, no matter how many hobbies you have. Employers don’t like this because that’s a lot of work they could have persuaded an employee to provide as unpaid overtime labor instead; much this outrage is simple jealousy. If you’re doing the…
The math doesn't work long term. It may be kept for 1-2 months even when a person is 21 yrs old, but I doubt it it can be sustained more than that.