When two of the Smithsonian Institution's pre-eminent museums reopen on Saturday after six years of renovation, visitors may be stunned to learn that they were once competing installations with little more in common than the subdued building that housed them.

The American Art Museum and the National Portrait Gallery, joint tenants of the venerated United States Patent Office, now share an interior reimagined with wider exhibition space, brighter light, soaring arches and common entrances for their intersecting interpretations of United States history.

The $300 million redesign, paid for by Congress and private donations, has left the museums separate in name only. A proliferation of offices and interior walls that once narrowed viewing space are gone, giving way to floor plans that sweep visitors from one museum to the other and back again. On the first floor, for example, the American Art Museum occupies the west side, and the Portrait Gallery the east. On the second floor they switch positions.