Answer
Most companies would make you sign a form declaring that while you work for them, you will not have any other outside business activities that would conflict with whatever they hire you for. for example, you cannot
work for microsoft during the day, and get a part-time job at apple working nights and week-ends. conflict of interest, your exposure to trade secrets that you may accidentally share with a competitor, or secrets you may find out about a competitor that could make you liable if anything goes wrong. one cannot
sell products for two different companies, it gives you no credibility with customers. it is
morally and ethically wrong, and with the heightened security and random background checks, one might get away with it for a while, but you will get caught.
