It may be best to do these procedures separately. One reason is that you may not able to achieve optimal results since the positions you are instructed to sit/lay in during your recovery are different for the two procedures.
For the Brazilian Buttock Lift, patients are advised to lay on their stomach and sides, and only on their backs for a short amount of time. Too much pressure to the area can hinder the survival of the fat-graft.
For the Tummy Tuck procedure, recovery is generally most comfortable in a recliner like position, which requires patients to lay propped slightly upright on their backs. This helps to protect muscle repair and keep the skin flap tension free, for optimal final results.
As you can see, the procedures are a bit counter-intuitive when it comes to the recovery process, and may make things more difficult than they really need to be. Consult with your Board Certified Plastic Surgeon, and map out a plan to do both of these procedures within a time frame that offers both safety, and the best results.