I good investment is more preferably than kill your back on two or three jobs. The chances of failing on both due to exhaustion is more than becoming an expert in one job and earn more as a professional. Train one thing, become a master of it and set your own price for your work and experience. That is more stability than balancing between two jobs.
Unfortunately not all type of jobs are like that. If you work in service industry for example, being the best ever waiter couldn't really bring you to a great level, you can't really become great at it and then start earning a lot. Sure, maybe you get a lot more tips than other people if you are lucky but that's about it, it is not enough at all.
I personally believe that having a second job that has a skill set that can scale to a high level is a lot more important. Most obvious and common example of this is coding, I am not saying learn how to code, but have a skill like that, which you could scale to millions of dollars if you are great, and even if you don't, at least earn enough to supplement your income.
Some people don't understand that we can do side job where we can earn more by using set of skills, but these set of skills are unique that should be invested, researched and committed. I have a part time job where I only spend 3 hours every day and my salary is more higher than the minimum wage earner in my country. I do not want to exchange my time for small salary because I have unique skill sets that can give me more opportunity. It is really good to find a side job that is really suitable for you, just make sure that you can still have time and financial freedom if you apply for your side job.