A lot of employment history is sold by the employer to "identity" services like Equifax, which runs a product they call 'The Work Number'[0] to allow other corporations to run background checks on you and see exactly where you worked and how much you were paid every month. While not all employers currently sell employee data to Equifax et. al, more and more will in the future and there's no guarantee they won't sell past data.

So I think it's going to get increasingly difficult to secretly work two jobs.

OTOH, I've negotiated to have two jobs at the same time, bringing one from full-time to part-time and joining the other part-time. Then I was able to work both jobs for 35 hours/week and make nearly double money. Both companies were okay with this arrangement, although neither knew that I was working both jobs nearly full-time. An open-source tool to sync just free/busy status between the two outlook calendars allowed people to schedule meetings with me as-usual, as events from the either company would be automatically copied to the other -- but only as "busy", nothing potentially confidential. [1][2]

It's interesting, because I would have been happy to be paid hourly to work 60 hours/week at either one of the jobs but neither would let me do that.

0: https://theworknumber.com

1: https://www.outlookgooglecalendarsync.com

2: https://syncthemcalendars.com/faq