Intro
Most people say you should have more then one source of income in case one of the income fails, you can have other income to fall back on. I will talk about when it is wise to have one source of income and when it is wise to have multiple source of income.
When you should specialize
You should first specialize in one source of income like a business. Put all your effort into growing that business so you can be the very best in that field. People pay more for the best. People will pay the top dollars for the very best then someone that is average. You can have more then one income like investing but don't spend too much time on it. Send 80% of your time, energy and money on that one source of income like your business.
When you should generalize
After you have made millions on your business, then you can have multiple streams of income. You can use excess cash and buy company's, invest it, etc. This is when the business is doing so well you can hire someone else to do the work for you and then you can focus on developing a new stream of income.
Conclusion
You should specialize and then you should generalize. Focus on one source of income and when you have a lot of excess cash, then you develop your other streams of income. This way you can make more money then those who have multiple source of income from the beginning.