Board Thread:Show-Related Questions and Answers/@comment-24304025-20141219181020/@comment-25355646-20141220000853

It was said in 3x19 that ever since David woke Snow from the sleeping curse, they "have been of one heart." I think this means that under normal cirtcumstances, if an enchanted heart is put inside a dead body, that person would live but the heart's original owner would die. But because of the TLK with the sleeping curse, there was enough reinforced light magic in Snow's heart for it to be split and be strong enough to keep both her and David alive.