(Looking through my notes - Vactyr and Delphteans already canonically have two hearts)
Oh, Vactyr probably have two hearts due to Leonyx ancestry. While they abandoned the multi-limbed body plan for a bipedal one, I could see it being easier to keep some of the extra organs than put in the Flow to remove them.
This means a Vactyr can take a serious shot to a heart and survive (for longer than a human would. Long enough to get help at least.) Non-friendly Vactyr fights turn brutal due to the horns.
Delphteans, though, don't share this due to the fact they have gills and lungs also sharing space there. It is a semi-common disability for Delphteans to lose the ability to be amphibious with enough trauma to the chest (I can see this being a possible birth defect as well). Most often, they lose the ability to breathe water well (they'd still have their cervical gills, but the thoracic processes the most oxygen), but sometimes they'll lose the ability to breathe air.
Lutskyllans don't have that problem, despite having both gills and lungs as well. Theirs are more sturdy and less prone to issues.