This has been a bug for years, and this is not the first time I’ve encountered this bug. I found this old thread from 6 years ago: Re: Can't have more that one part time job (same time, different days) | EA Forums - 11424605. I don’t play with part time jobs that much, because it ends up being frustrating when I do.
One of my currents sims has no problem working as both lifeguard (Thursday-Sunday 08:00-12:00) and babysitter (Saturday-Sunday 18:00-00:00), or even babysitter and handyperson (Saturday-Sunday 07:00-12:00). even though two days overlap. But the same sim can’t work as both babysitter and diver (Monday-Friday 17:00-20:00), even though the days do not overlap. The only overlap is time, but it shouldn’t matter when it’s not the same days. If I employ my sim as a diver, my sim has to resign the babysitter job.
Edit By Crinrict: Archived thread had around 4 me toos, adjusted title to fit.
