Patients having two joint replacements at the same time is a controversial question. In general, most patients are best to have a short hospitalisation, get out and get recovered from the first side, before embarking on the other. Indeed most patients have substantially more symptoms on one side than the other.
There is a small number of patients that are troubled by both sides, and each side would hold the other back from recovery. For example, if there is a substantial fixed deformity (i.e. stiffness altering gait pattern) on both sides, replacing one only runs the risk of it not regaining a normal range of movement. This seems to apply equally to both hips and knees. None the less, the patient needs to be in good physical shape. Severe immobility prior to the surgery and a lack of fitness would be an indication that inpatient rehabilitation might be required. The patients in good shape however can expect to be home in four days or less.