How am I supposed to sleep after having a tummy tuck and a BBL done at the same time?

The benefit of staging is ease of recovery. The benefit of stacking is the convenience of a single procedure which may represent a time or cost savings.
There is a difference between procedures which "could" be performed simultaneously and those which "should" be performed simultaneously.
While they can be performed safely during the same procedure (assuming a 6 hour max OR time), some consideration should given to the nature of these procedures and the anticipated recovery period. In general, it is always best to minimize the pressure on the surgical site. The recommendations for these 2 procedures in particular are in conflict with each other (one necessitating prone (belly) sleeping and the other supine (back) sleeping).
There is no perfect solution in this scenario. For this reason, I do not regularly combine these procedures. Side sleeping is sometimes a reasonable compromise. Alternatively, patients can rotate position frequently (on the hour) or use a specialty pillow to offload the buttocks grafted area. However, for best results staging or staggering these procedures is best.
With regards to your specific question, stacking these procedures can be safe, staggering them is relatively more practical/sensible (when one considers post op recovery). If one can make the necessary accommodations for recovery, the procedures should have no effect on each other. The mattress may be of help, however, it is up to the patient to decide of these accommodations can be made.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Unfortunately the only way to sleep would be on your side as you don't want to pressure on your buttocks or on your stomach. You may need to use pillows to prop yourself into a position to ensure you don't turn onto your back or stomach. This problem is the main reason I recommend my patients to stage these two surgeries, doing the BBL first and then having a tummy tuck 3-6 months later. Good luck!Kamran Azad MD, Board Certified Plastic Surgeon

Your surgeon should have a protocol for sleeping when going thru multiple procedures like this. Some patients choose to split these procedures up - but that is your discretion following a consultation with a board certified plastic surgeon. Good luck-