The family-owned New York chain tests a brand-new, bridal-only concept at its Two by London spin-offIf the recession taught the jewelry industry one thing, it's that people continue to get hitched even when times are tough. The wedding biz held its own during the worst of the downturn, and in an age when diversification reigns, retailers have responded by playing up bridal-only extensions, both on- and offline. Two by London is testing the bridal-only tactic in a big way.A bridal-only business idea came to Scott and Jessica Udell—fourth-generation husband-and-wife owners of London Jewelers, an upscale five-store chain based on Long Island—when they learned many of their marriage-age friends assumed they couldn't afford a diamond from their family's fleet of boutiques. The situation called for rebranding: They wanted to create a modern venue for a younger clientele stocked only with e