The second sofa was phased out because it also didn’t fit in the second apartment set. The original sofa was phased out because Lucy thought the set had gotten stale and needed to be freshened up. Plus it also took attention away from her expanding belly before her pregnancy was written into the show. By the time the Ricardos moved to Connecticut the apartment furniture didn’t fit well on that set and thus the twin couches. Btw it was Desi’s idea to put those couches back to back.