Germany: It is part of my contract that my employer knows about my other jobs and they don't interfere with my actual job. E.g. having a side business that you maintain outside your working ours is fine.

You also can't have two full-time jobs without you violating labor laws or defrauding your employer. Since if you aren't defrauding one employer you would have to work 16 hours a day, which you can not do for 5 days a week in a row. And there is no way to actually hide that your are doing this. Employers are forced to pay certain taxes for employing you which can't be payed by two different companies for the same thing.