It is very common to have these two procedures performed at the same time. In fact, it is so popular in my practice that we have a nickname for this combination: The Mommy Makeover. It is my experience that most women don't want to look like Pamela Anderson but rather they just want their pre-baby body back.
The mastopexy (breast lift) is generally performed first. During this portion of the procedure, excess skin is removed to relocate the nipple-areolar complex and to reshape the breast. The objectionable sagging is corrected. Next, the abdominoplasty (tummy tuck) is performed. At this point, the abdominal muscles are tightened and excess skin (usually containing most and possibly all of the stretch marks) is removed.
Of course, every patient is unique and has her own set of circumstances and health issues. A complimentary consultation would be essential to review these. Each patient's treatment plan is personalized for them during the consultation.