You cannot work for another person while running your own business. A business becomes the centre of a person's universe and an employee's work will suffer if they are building a genuine business on the side. No amount of dramatic profanity will change that reality.
Of all the vigorous down-mods and "you're wrong!" comments I've received here in response, I've yet to read a single real-world example of a person who has a full-time employee on staff working on their own business, or who is a full-time employee working on their own business while being paid a salary by another company.
Why? Because you're an employee or you're an entrepreneur. Those are not compatible mind-sets.
If you work for me and want to start your own company, great! I'll happily send you on your way and offer every bit of support I possibly can to ensure your success, but I won't pay you to build your own business.