This is not a question agreed-upon by all plastic surgeons. There are good plastic surgeons who will insist on doing the procedures separately and there are good plastic surgeons who can produce excellent outcomes in a single stage.
The combination breast augmentation / mastopexy surgery differs from breast augmentation surgery alone in that it carries increased risk compared to either breast augmentation or mastopexy surgery performed separately. Furthermore, the potential need for revisionary surgery is increase with breast augmentation / mastopexy surgery done at the same time.
in my opinion, the decision to do the operation in a single or two staged fashion becomes a judgment call made by a surgeon after direct examination of the patient. For me, if I see a patient who needs a great degree of lifting, who has lost a lot of skin elasticity, or whose goal is a very large augmentation then I think it is best to do the procedures in 2 stages (in order to avoid serious complications). However, doing the procedure one stage does increase the risks of complications in general and the potential need for further surgery. This increased risk must be weighed against the practical benefits of a single stage procedure (which most patients would prefer).
Conversely, if I see a patient who requires minimal to moderate lifting along with a small to moderate size augmentation (and has good skin quality), then doing the procedure one stage is much safer. Nevertheless, the potential risks are greater with a 1 stage procedure and the patient does have a higher likelihood of needing revisionary surgery.
I hope this helps.