Can a fat transfer to the buttocks and breasts be done at the same time?

Both these surgeries can be performed at the same time but you will need enough fat for both procedures. This typically means about 1000 cc of good processed fat at least.

To answer this in sequence:
1) surgeons vary as to their recommendations with most advising two weeks off of your buttocks. However, some as you indicate, may suggest longer periods.
2) these surgeries can be performed at the same time but you need adequate & sufficient donor sites as well as breast augmentation through fat injection in the USA remains in its preliminary stages of evaluation.