Whether or not you are permitted to work two jobs depends on your work permit type.
As far as CEC program requirements go, any work experience that is 30 or more hours per week is simply "full time," whether those hours are gained in one or two jobs.
So the extra 15 hours per week on top of 35 hours per week will not add anything to your eligibility. In submitting your online EE profile, it would be OK to list only your supervisor experience, but later, if you get the ITA, you will have to include ALL jobs on your application form.
Just a word of caution, it can sometimes be difficult to get "supervisor" experience recognized (read the job duties of the NOC very carefully). So if your experience at the tax firm is skilled, you might want to consider using that. You don't say how long you've been working there, but 15 hours per week * 2 years would meet the 1 year requirement.