My first question is why are you having back to back treatments? Some people achieve the result they want after a single treatment, particularly thinner people. To maximize results, be certain there is adequate massage performed. This is what helps to break down the frozen fat cells. The fat is then processed by the lymphatic system. This is why in my practice we incorporate vibrational therapy and both manual and mechanical massage.
If you mean two areas treated back to back, that is perfectly reasonable and will not increase the possibility of bruising. Some people bruise. others don't. Some people bruise following some treatments and not others. It is unpredictable. We always ask patients to be prepared for some bruising.
Post surgery patients receive arnica montana to reduce bruising, but you do not want to use arnica following CoolSculpting as it may interfere with the results of the procedure.