How? If I'm willing to work another job while I'm at my first job, no one would know if the code I'm working on my screen belongs to job #1 or job #2.

The employer could use screen monitoring, but then they could do that while I'm at home and see that either I'm working on a second project, or that my computer is idle for long periods of time when I'm supposedly "working".

Though I'm not sure it matters - if I can work 2 jobs and still provide adequate productivity to each employer, then why do they care?

I have so many meetings every day that I'm sure that I couldn't get away with working 2 jobs at once, even when fully remote.