Well, adjoining means "as close as possible" in room-assigner-speak, so that's not a difficult request for them to fill. In practice, "close as possible" could mean next door, across the hall, down the hall, or just in the same building.
Or are you looking for connecting rooms? Rooms with an internal door between them, so that you don't need to go out into the hall to go between rooms? If that's the case, yes, it's possible to not get connecting rooms if you request them. Disney doesn't block connecting rooms - in other words, when you request a connecting room pair, they don't put that room aside for you. So, you can have more people requesting them than there are room pairs at the resort.
Most people who request connecting rooms do get them, and if they're available, you'll get them.