How are people “over employed”?

Has anyone ever tried being “over employed” – i.e., working two or more remote jobs at once? How do you make sure one job doesn’t find out about the other?

I feel like your original employer would find out incredibly quickly just based on work output right? Also, how do you manage meetings? I feel like you can only say no to meetings (because you’re double booked from your other job) a few times before someone catches on.

I think one of my coworkers is over employed, but I’m obviously not trying to ruin his gig if that’s truly the case. I’m mainly just curious how people are able to handle that.