Twin Reversed Arterial Perfusion (TRAP) Sequence is a rare complication of monochorionic multiple pregnancies. It happens in less than 1% of this sort of pregnancy.
The heart of the healthy twin does the pumping for both babies. It is called the "pump" twin.
The acardiac twin unfortunately does not develop properly. It has no heart, and cannot survive outside the uterus.
There is no consensus on treatment for TRAP Sequence. Some centers will monitor closely, watching for signs of compromise with the pump twin, and others will choose to cut off the cord flow to the acardiac twin.