Can you put two upstairs and two mountain and sea towns downstairs in the duplex building?

No, it's just like putting eight diagrams, no more than three. Shanhai Town is divided into indoor use and outdoor use, but they are all outward. I don't know what kind you bought. Outdoor ones should be placed on both sides of the door. Indoor ones should be placed on the opposite side of the door (without affecting access) facing the outside. The indoor mountain and sea town should be placed in a suitable position with the entrance facing outwards, so as not to affect people's access. Remember not to point at other people's doors, or their luck will be stifled.