Last summer, Kathryn Swimm and Kathleen Mullins decided to meet in the middle. Ms. Swimm was living in a “dungeon-like” junior one-bedroom in Yorkville, on the Upper East Side of Manhattan, and wanted to upsize; Ms. Mullins was in a three-bedroom flex with four roommates in Turtle Bay, in Midtown East, and wanted to downsize. “But nobody moves to an …
