Depends what sort of hours you work. If you were doing 9-5 and had no chance of doing study during work time, I would only do one course/semester. If you only work (fly) 12-14 days per month, or spend most of your office time on FB or solitaire, then two subjects is achievable.
Best option - enroll in one, and judge the commitment for yourself. You can always take on more work...