that's not really how it works yknow. A business pays for your time, int he fashion of an hourly wage or a salary, they DO NOT own your off time. With that you can relax, vacation, OR work another job. What difference does this make if you start a side business or work a second job?
As long as it is not in a competing position to your main employment, it should be all good (example a mechanic should not be running his own shop on the side, however there is nothing wrong with being a mechanic for two different businesses... if one of the businesses doesn't like it, they can simply keep you busier/pay you more)....
MANY people have TWO part time jobs, or a full time AND a part time job... how is it different from working as a car dealer service manager and mowing lawns on the side?