The only correct answer to your question is "it depends." In the past two years, I've started three consumer products companies. During this time, I've always worked on at least two simultaneously. Meanwhile, I'm also currently writing a book and sitting on the board of a new non-profit organization. All in all, I've never been one to go by the rules of focusing on one business idea at a time. You can argue adamantly for both sides of this. Personally, I go by the theory of "don't put your eggs in one basket." But honestly, it really depends on what type of business(es) you're starting up.

Throughout my time as an entrepreneur, I've learned a ton of lessons on which tactic works best, which business ideas to pursue, and how to validate your idea before putting all (or even some) of your energy into it. I'd be happy to evaluate your business ideas and lifestyle to determine what approach might work best for you. Would you like to do a call sometime?