How safe is it to have breast reduction and a tummy tuck at the same time?

It can be. Surgery varies in extent between patients. As long as the two operations are not quite extensive and the patient is healthy, doing them concurrently can be safe. It can make recovery a bit of a bigger deal though. :).
Generally safe. It is generally safe, though surgeries do have a little more risk when they are over 6 hours, such as slight increases in DVT risk. If your surgeon has an assistant, the two can be done in a reasonable time.
Safe w/ Precautions. It is safe with precautions. These are 2 major surgeries done at once. There is increased risk of blood clots when another procedure is combined with a tuck, but these 2 procedures are commonly done together. Make sure you stay hydrated the week b&a surgery, ask that pneumatic stockings be used on your legs during surgery & post op if possible. Use compression hose post op and walk frequently.
Yes but some risks. This is a quite common combination of procedures but they vary tremendously in extent with some as short as 3 hours and others taking well beyond 6. I would discuss the procedures and your candidacy with a board certified plastic surgeon with experience.
Me: not recommended. Breast reduction is all about pushing up and abdominoplasty is all about pulling down so they can be difficult to combine. These can both be sizable surgeries and so the risk would not be worth it in most cases. Small reductions might be combined with a small abdominoplasty but giving the body too many big areas to heal at once is never a good idea, in my opinion.
Relative. This all depends on your health and the skill of the surgeon. Both of these are major surgeries and have complications that can be associated with them. The longer you are under anesthesia the higher the risk. That being said if you are healthy, a non smoker and not significantly overweight it can be done.