I don't do it often, but anywhere from once a month to once-every-two weeks, I will do a double workout on one day of the weekend. I usually do strength in the morning, then cardio in the late afternoon. Sometimes, I won't finish my total body strenght, and leave biceps and triceps to do with the cardio. It helps me put more energy into the arm exercises (which I sandwich in between the cardio warm-up and the rest of the routine).
After seeing my profile in a full=length mirror in a hotel (worse than dressing room mirrors!), I'm tempted to up the frequency of these double workouts to once a week or once-every-two-weeks to blast away that belly bulge!