The Dolphin, Swan, and Swan Reserve are sister hotels that share a website, a single local phone number for reservations, and even their own app. You can charge meals at one hotel to your room in the other - restaurant bills all have a place to check which of the three hotels you are staying in. I mention this simply because that might make this situation different than the usual situation in which a local Marriott would not know you also have a reservation for a same night at another hotel. If you are concerned that the Dolphin reservation might get cancelled if someone sees you already checked into the Swan Reserve, you might want to give the Dolphin a call directly to alert them to the situation.