I don't think "shadowing" happens much, but the best specialists interact and know about related fields. The rad-oncs and heme-oncs often have joint meetings where they discuss cases and they coordinate care of their patients to get the best results.
The best radiologists are those that learn as much about the clinical management of certain conditions as possible. Whenever neurologists review a scan with the neurorads, there is usually a give and take as to the best management of the patient and both sides usually learn a lot. The same goes for surgeons and abdominal radiologists, thoracic surgeons / pulmonologists and chest radiologists, etc. At least in academic centers, there are meetings involving radiologists and clinicians in nearly every possible field where the imaging and clinical management are discussed.
Whenever I call something on call that results in action (surgeons taking a patient to the OR for an appendicitis or internal hernia that I called on CT, OB/GYNs taking a suspected torsion to the OR that I called on US, etc) I will either directly follow up with the service or at least read the notes in order to learn what they saw during the procedure. Its a great way to learn.