I've never had to work two jobs at once, but at various points I have:

1) Studied/spent time in class for upwards of 70+ hours a week in University,
2) Worked full time, while studying for my accounting designation,
3) Worked a job which had me working 45+ hours a week for trout pay which wouldn't be so bad in and of itself except I had to work until 1am every night and still had to get up at 7am the next morning for other things.

All of these situations have been accompanied by various levels of burnout and/or depression. Studying in University wasn't so bad because it was usually just a result of finishing up projects during the last couple of weeks while starting to study for exams and still go to class. It was usually 2-3 weeks then it was done so there was always light at the end of that tunnel. Studying for my designation was less enjoyable because it went on for more than a year. There were breaks between courses here and there, but they were usually filled with more work at a place I hated, so that didn't really help. And the last one was at a call center, so I hated my job and I wasn't sleeping.

Long story short, if a second job is going to mean you don't get enough downtime or sleep, then burnout is going to be a big risk obviously. If you absolutely need the money then there's really no choice, but if the money is more of an added bonus than at least you can try it and quit if it's too much to handle or you hate your second job. Ideally, if the second job is something you actually enjoy then it may not be so bad. But it can be tough otherwise. I really don't think most people are wired for working much more than 40 hours a week to be honest. It's hard enough to get a lot of time to yourself when it's just a full time job and a relationship on your plate.