By convention, a surgeon should restrict himself to the speciality in which he has trained. Thus a neonatal and paediatric surgeon should restrict himself to neonatal and paediatric surgery.
However, most surgeons have a basic qualification in general surgery before they embarked on training in a speciality. Thus, legally, such a surgeon, with a qualification in general surgery, can perform surgery on adults.
Not many are aware that a doctor with the MBBS qualification is a Bachelor of Surgery and can, thus perform surgical operations. Most doctors with MBBS refrain from surgery as they restrict their practice to family medicine.
The doctor with MS, MCh qualifications can certainly operate on patients with urological problems, especially if the specialty qualification is in urology. Many general surgeons, with the qualification MS, remove enlarged prostates and operate on diseased kidneys with excellent results.
Obviously, a person with MCh in urology will have greater understanding of surgery on the urinary system but a senior general surgeon may perform as well as - and sometimes much better than - a young specialist.