Should I get mentoplasty and rhinoplasty at the same time or separately?

Same time. A plastic surgeon always looks at overall balance when deciding on an operative plan, particularly when it comes to these procedures. This is a very common combination of surgeries that complement each other very well.
Depends. If both operations are indicated, they can certainly be combined. See several surgeons certified by the american board of plastic surgery or a board certified facial plastic surgeon to obtain several different opinions about whether one, the other, or both operations are indicated in your particular case.
Same time. If you are a candidate for both procedures and want to have both done it will be cost effective to have them done at the same time. You will avoid two separate periods of downtime as well as duplicate anesthesia and facility fees.
If needed, same time. The combination of rhinoplasty and mentoplasty can be transformative. They can be safely done together, and indeed, it is a common combination. In fact. Doing them separately may even cost more and certainly would leave you with an overall longer recovery time. Unless there is some overwhelming reason, doing them at the same time makes more sense.
Same time is best. If both surgeries are indicated it is usually best to combine the two procedures. This allows you to have just one recovery period and it ends up having lower total costs in the end.
Same time. Most of the time the mentoplasty adds on a short time to the operation and there are usually some cost savings involved in doing both simultaneously. Also, the menoplasty is usually a nice compliment to rhinoplasty and visually makes the nose look smaller and better proportioned.
Rhinoplasty and chin. Rhinoplasty and chin augmentation or reduction is commonly done together. They are absolutely safe to do as a combination.