If someone betrays your trust or doesn’t perform the job adequately: fire them. If someone performs the job adequately and can hold down a second job simultaneously then what is the problem? If you’re concerned you’re not getting value for money either extend the scope of your job until they can no longer hold down two jobs simultaneously and have to choose one or the other or fire them and hire a less competent employee who takes twice the time to do the job and costs less. But both of these options seem pretty ludicrous. Honestly, the entire debate around this subject all just stems around ego and power/control driven managers who shouldn’t be in the job in the first place.
And no, I’ve never personally held two remote jobs simultaneously. But companies really need to move away from this “paying for time” mindset to “paying for the job” mindset. Like if they want to do remote and they’re concerned about this then why not just hire contractors?