Safe but....

It can be done safely and is common practice among surgeons.  For better of for worse, combination surgery came into vogue partly due to television (the Swan, etc).  The reality is that the longer you spend on the operating room table, the more chances you have of complications.  That is true of all forms of surgery, not just plastic surgery.  I did an extra fellowship in Pediatric/Craniofacial surgery, and the longer the kids were on the table, the higher probability of complications, particularly with cranial vualt remodeling.  All that aside, there are retrospective clinical studies in the Aesthetic Surgery Journal, for example, claiming that multi-site surgery is safe.  One of the key issues to mention is that none of the patients spent more than 4 hours on the table.   So, is it safe?  There is no clear answer.  Having multiple surgeries simultaneously increases risk but so does two separate anesthetics with two separate recovery periods.  The most important issue for you is choosing the right surgeon.