You can certainly have multiple jobs while having a full-time job. But if you left your full-time job to pursue several careers with uncertain prospects for pay, then having some income, however modest, is a huge confidence boost. Any job will do. Remember, there is dignity in all labor.
When you leave a regular job to pursue your interests, you are likely full of confidence and excitement. That's good. We have to protect and capture our moments of inspiration. But all of us will surely face difficult challenges in our new careers. Even if you have savings, your confidence and excitement will dip at times as you make mistakes, face rejection, and face the realities of making a living.
It's especially important to give your motivation and confidence a boost every now and then when starting a business because there are so many things to do and the outcome seems so uncertain. Having some income stream, however small, will give you reassurance and confidence during difficult times.
And no one is immune from difficult times. There is nothing wrong with difficulty. The problem is wishing you will never face any. Since difficulty is inevitable, prepare yourself by keeping busy and having some income.