The technique of heterotopic (as opposed to the more conventional orthotopic) cardiac transplantation involves the anastomosis of a donor heart alongside the native heart, resulting effectively in a double heart.  

It is thought to have advantage in a select group of transplant patients. In particular, it may be performed in patients with refractory pulmonary hypertension, which would otherwise preclude orthotopic transplantation due to the risk of donor heart failing as it is unable to cope with the elevated right heart pressures.  

It also allows transplantation when there is a mismatch between the patient size and heart size (large donor, small heart). In this situation, the native heart takes on an "assist device" role, contributing to the total cardiac output, and potentially supporting the circulation during the initial adaptation, and at times of transplant rejection.