I did for a while. I had a full time work hard job, with awesome pay and no medical benefits.
I also had a full time lifeguard/swim instructor job with horrible pay and awesome medical benefits--non profit. Now the lifeguarding part was about 30 hours in the week, and honestly I just sat on my butt watching one indoor pool that was only for adults and only 4 ft deep that was normally empty. Not that I didn't watch the water, but I can't imagine a less intense lifeguarding job. On Saturdays I would teach 10 hours of swim lessons--no need for a gym membership (I was in the water the whole time with little ones--10 hours of treading water (same place different pool) and using my outside voice).
I had Sunday off from both jobs, and would seriously sleep until 1 pm. This was before direct deposit, and for a while I didn't have time to go to the bank---I had like 9 paychecks sitting in a pile. Eventually, I added my mother to my bank account and she would go deposit checks for me twice a month. I saved up a ton of money and paid cash for a 3-flat. I ended up essentially quitting both jobs at the same time. I got a great new job with good pay and benefits. I still continued to teach swim lessons on Saturdays I just cut back to 5-6 hours, that way I could walk at the end of the lessons.
OP-If you are going to work 2 full time jobs. I hope one is a non-physical job. I don't know you or where you live, but can't imagine two heavy factory type jobs. At the same time if you have two office type jobs, when will you get any physical exercise. I need to know more about the jobs to give you an honest opinion. I know I would not have been able to keep both jobs for as long as I did, if the lifeguarding job was strenuous on a daily basis. Also not working either job on Sunday, helped a lot.