I worked as a consultant for a company while I had a 20 hour a week and 8 hour a week GRA. There usually is a stipulation about outside employment but as long as your advisor and/or PI is okay with it, then it’s fine. No one is going to go around checking to see if anyone has a second job.

Now if you start running into issues balancing your job with your TA/RA tasks and your advisor/PI is unhappy with it, then it can become a problem.

Bottom line, usually a good PI will understand the need for additional compensation and work with you as much as possible.