I've never known anyone in that type of situation, so feel free to disregard what I say here.
It's been my observation that law jobs do not suit themselves well for set time commitments. They are often unpredictable. So although someone thinks they'll do doc review for 30-35 hours per week, it could end up being 50 hours some weeks and 0 hours other weeks. And to the same degree, although someone commits to a minimum of 20 hours per week at a nonprofit, there may be days or weeks in which the employee will not be able to finish the amount of work expected within 20 hours (or at least the employee will not be able to do a good job in 20 hours).
Law jobs are often project-based and deadline-oriented. That is, you are expected to put in however many hours are necessary to finish the project by a certain deadline. You often can't just punch in and punch out and say "I'll finish it next week."
So, I don't think it would work out very well to have two law jobs and expect to put in a set number of hours at each. I think the employee might be setting himself up for failure at both jobs.
But again, I stress that I've never been in that type of situation, so I don't know from firsthand experience.