Island Sanctuary has an ability whose cost is to skip drawing a card. The ability is therefore played as a specialized ability, just before an effect would have you draw a card. If you play this ability, the card draw is replaced by Island Sanctuary's effect. That series of specialized abilities ends, and another one begins, to give players the chance to modify the new event. Other abilities that modify or replace card draws, such as that of a second Island Sanctuary, can no longer make use of that card draw.