I'd ask your Ambassador whether it's possible to arrange a pre-upgrade at one or the other--allowing you to apply your SNAs at the other. As this is for a special occasion, your Ambassador and the hotels likely will want to try to do what they can. Having stayed at both hotels and having been upgraded at both on special occasion trips (Gritti via SNAs, StR Florence via comp pre-upgrade thanks to my Ambassador and status), I trust that both will do their best for an Ambassador elite.