How do I import one Unity project in to another Unity project?

I built a room, Project1, and separately built a tabletop display/diorama (Project2) that goes in the room. In each project, the model shows correctly in the Assets window for that project. But, when either project is open, the other Asset is nowhere to be found. How do I get the two projects open in the same window? Thanks. Salut.

problem solved. Salut. pmj