This hotel has single only rooms, so if you book that room I think it's likely they would charge you the regular extra person charge (~70-100EUR in my experience) plus the cost of the higher category room since the single bed room doesn't legally accomodate 2 people. However, if you were pre-upgraded to a larger room that accomodates 2 people then you would probably just get charged the extra person fee or maybe nothing at all. If you're platinum I think the latter case is the most likely.

I made several award reservations (which are max 2 adults) at various Starwood properties across Italy, and most hotels charged us around 80EUR for an extra adult because we had 3 adults. One hotel didn't charge anything and one made us pay an extra person charge plus 50 EUR for a deluxe room that would accomodate 3 adults. I think the SR Florence charged 85EUR, but can't remember exactly.