When a BBL (fat transfer) is done with the rigth technique, you can sleep on your back from the beginning, using the appropriate belt. While with the breast lift, you should always sleep on your back for a period of one to two months. But, if gluteus implants were used with the BBL, then you would need to sleep facing down for 15 days and would not be able to perform the breast lift at the same time.
I usually have my patients lie on their side when dong both and frequently change position as well...