Thanks for posting on HCM,
I sympathize with your situation and I know it must be difficult right now, but be positive as the other twin is apparently still in good shape.
One the greatest cause of death of one twin is a complication known as twin-twin transfusion syndrome. Here the intervascular communication between the mother and the both babies is in such a way that one of the babies is favored (in terms of blood, oxygen
and food supply) where as the other twin is deprived. The vascular communication is usually between both babies, there by making that one baby received every supply from the mother, while the other receives little or nothing from the mother. This arises to poor growth and development ( already noted in previous ultrasound
) where as the other twin grows even as twice the normal size of a baby with healthy organs ( such as heart, lungs etc). The previous doctor normally would have told you about the condition. There are several treatment options available for twin-twin transmission syndrome ( so needed to consult with a gynobs for treatment options), even though must of the treatments are still under study.
Hope that this helps