Everything has a cost. Working two jobs will cost you some rest and relaxation, maybe some time away from your hobbies. Working only one job will cost you the money you might make working the second job.
Sometimes doing something different for a period of time can be invigorating and stimulating, rather than just tiring. I am currently working on a little side business that is only peripherally related to my main business. Yes, it's more work, but there's satisfaction there, too.
If you're 31 and working for minimum wage, first, I applaud you for working, but secondly, you should be looking toward the future. Either work on learning a trade which will be worth more money to you, or perhaps work on moving up at DD. They'll need a shift lead eventually, maybe an assistant manager, and so on. When I was a teenager working in fast food, I just showed up, did what was required, and left. Another guy showed up early, left late, and generally did a better job than anyone else. He became the assistant manager, then got hired at Burger King as a store manager, and not too long after that (maybe a few years), went to work for BK corporate. It can happen.