Responses
One of them belongs to the Monarch Of death which that statue angel is trying to revive , the plan was upon giving Jin Woo the shadow sovereign memories he would take over Jin Woo's body but that won’t happen the two hearts is just them being one and the same so the both of them are sharing the same body but the shadow sovereign is currently in a hibernated state

Don't me him having 2 heart means the human heart is from his mother and the other one he inherited it from his dad? Does it mean that his dad is not human?