I was talking to a person today that worked a full time business while also having a side business income. His idea for it was interesting as depending on his mood he actually switches to full time efforts to his other business. The mentality was this way he will always have a choice incase one industry becomes cold. Likewise, if another industry is hotter at the moment then he can switch and get more profit from that one.
You must really have to know your stuff to make that work as the amount of research and knowledge you would need to anticipate changes like that must be pretty high. Kind of a twist in many ways too to those that work a regular job and then trying a small business on the side. I guess online businesses would be easy to switch though if your business is mainly about selling products as it’s mainly about different inventory while following a similar system.