Even taking on a side job contract has IP issues. Most companies claim full ownership of any IP produced by employees at any time, on any devices.
There’s also workman’s comp and medical insurance, unemployment insurance and all other issues that conflict with multiple full time employment.
I know people that work full time and do a side gig. Either their own business or a part time under the table contract job.
I’ve never known anyone work multiple full time SE jobs. That leaves easily discoverable paper trail.