Thank you for your question. Generally speaking, I prefer to perform at tummy tuck and BBL separately to optimize healing. Since you have to lay on your back after a tummy tuck, you won't want to be worried about jeopardizing your BBL results. You should have a consultation with a board certified plastic surgeon for precise recommendations.
I don't do these at the same time. While they certainly "can" be, a better question is "why" - considering all the difficulties/risks involved I don't recommend it.
You technically can do both of these at the same time and we do - but it is a really hard recovery for the patient. You do need to sleep on your belly for about 3 weeks and this is really hard with an abdominoplasty. Some patients will sleep on their side but that is also not great if you have fat grafted to make bigger hips. Patients do have more discomfort and it seems that the recovery takes about 6 weeks rather than 1-2 weeks with BBL alone. That being said, you just have to weigh your options on hard recovery vs. 2 recoveries. Best wishes!
These are definitely two procedures that should be performed at two different times and with enough time in between for your body to heal. You are correct in that with a BBL, you must refrain from any type of pressure including sitting/laying directly on it for a couple of weeks. This is not advised post tummy tuck. Please see a board certified plastic surgeon to discuss options in person.