DUPLEXES HAVE COME a long way since the days of floor plans that resembled Rorschach ink-blot tests. Fueled by demand for higher density and a diversity of housing types in master planned communities, twin homes are making a comeback. But today's double-duty models are infinitely more sophisticated than their predecessors. More poetically referred to as “paired homes” or “two-family homes,” they seldom sport mirror-image layouts, much less matching elevations. In some cases, you'd hardly know they're related.