Should I accept an offer?

I am currently working as an audit intern and will more than likely be offered a position. My only dilema is that I have another internship with another firm during the summer. These internships are in 2 different service lines (audit/advisory). If I am offered a position should I just accept it and possibly rescind? Or should I be up front and ask if I can wait until after I complete the second internship to make my decision? I don't want to burn any bridges, but I also don't want to lose out on a job opportunity in the event the second internship doesn't work out or isn't a good fit. How should I proceed?