It can be done at the same time, but no surgeon would do like that. They prefer to do in one eye at a time. The other eye can be done after the eye that be performed heals well. It usually takes three days to perform the second-eye cataract. Some people may wait a month to have the other eye done.
In the US, most insurance companies won't allow the surgeons to perform both eyes at the same time. Cataract surgery can only be performed on one eye at a time. When to perform the second eye will depend on how well the first eye healed.
Technically speaking, there is no problem for the doctor to perform cataract surgery on both eyes at the same time. But for the patient's safety, it is not suggested to do it at the same time. Because eyes can get infected easily during surgery and if both eyes are infected, the result is quite serious. So to lower the risk of getting infected, cataracts surgery is usually performed separately.
Usually cataracts surgery in both eyes is performed separately to reduce the chance of getting infected on both eyes. After one eye is done, the doctor would observe how it react and recover. The second eye can be treated approximately 1-2 weeks later after the first eye has stabilized.