It takes 20-30 minutes before fat becomes the primary fuel source. In two 15 min sessions, you are still getting the cardio pulmonary benefits. The exercise also speeds up your metabolism and burns fat for a while after exercise, although this depends more on intensity. The higher the intensity, the longer and stronger the metabolic after burn.
What kinds of low impact aerobics are you doing? Maybe you could rev up the intensity for those two sessions?
Do or don't do; there is no try (Yoda)