SSA would probably decide that they were holding out as married and still consider them a couple. It obviously depends upon the details. If a spouse took in a homeless schizophrenic ex-spouse and let them sleep in the garage and use the bathroom, it probably would not be holding out as married. If the only thing that changed is the legality and not the sleeping arrangements, then SSA would probably consider them holding out as married.

Then there are the married couples that don't get along and consider themselves separated because they sleep in separate bedrooms and don't talk to each other. Well, for SSI, they are married and living together. Doesn't matter where they sleep as long as it is in the same house.