Can I get a BBL and a tummy tuck on the same day?

There is a difference between procedures which "could" be performed simultaneously and those which "should" be performed simultaneously.
BBL and tummy tuck can be performed during the same procedure, however, 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 contradict each other, with 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 is 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. From a positioning perspective, your buttocks should be offloaded but prone or belly sleeping is discouraged after a lift. Side sleeping is likely best along with a buttock pressure relief cushion.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

It is always recommended to have a liposuction with transfer to your buttocks first as this guarantees more fat to be transferred than when it is combined with another procedure . As a surgeon is only allowed to take only about 1000cc of fat in a combo procedure instead of 4000cc in a single procedure, 6 months after will be a perfect time to have a tummy tuck.
Surgeons, Like myself offer online consultation, so that you can be examined.Choose a board certified plastic surgeon who is highly experience in Brazilian Butt Lifts.  choose one wisely.