I believe back in the day, Marriott Corporate wouldn't allow you to combine the brands into a single property, so you'd have to build two completely separate properties even if they were adjacent to each other. Probably something about diluting the brand image. All of the shared facilities properties I've seen were built in the last 5 or so years.

For example, there's a Fairfield/Residence Inn/SpringHill on the north side of Orlando Airport. Each is a completely separate building with its own pool. There's also the Springhill/Residence Inn complex at the Orlando Convention Center. These are shoehorned together and each has their own pool right next to each other, separated only by a fence.

For new construction in Orlando, you have the SpringHill/Townplace on Palm Parkway which shares lobby (shared check-in), pool, and breakfast area. There's a Springhill/Townplace on the west side of Disney which shares a restaurant and pool, but still has separate buildings with separate check-in areas.