I always assumed that when someone says they are “working two jobs” it means they have a full time job, but times are tight so they have also had to take on a part time (or even sometimes an additional full time) job.
It is usually said with implication of extraordinary efforts.
Several times recently I have seen people write that they are “working two or even three jobs”, still with the context being that this is an extreme thing to do, but it turns out they are all part time, not even adding up to full time between them.
I guess that multiple part time jobs are probably more common these days with zero hours contracts, and I know it can be stressful trying to juggle the time commitments, but I don’t think it’s the same as working significantly more than a full time job. I usually had several simultaneous part time jobs as a young adult, in fact I didn’t give up my Saturday retail job until a couple of years after getting a “normal full time” job.
YANBU - “working two jobs” should mean more than full time.
YABU “working two jobs” just means having more than one employer regardless of hours.