It's possible. What arts majors are you interested in? If they are similar or by the same department, you can sometimes get a single subject to count towards two majors. There is a limit (I think it's 2 subjects, none at 300 level).
MQ will let you 'add on' a major if you don't have the elective space- but you need to fill out a form.
I don't know the rules for Commerce, but in Arts you should be able to double major.