Yoël and Michael Zaoui spent more than two decades as Wall Street investment-banking rivals, repeatedly facing off across the negotiating table as they advised dueling clients.

Today, frères Zaoui work side by side in a London office, and their eponymous firm, which they launched last fall, has shot to prominence. Zaoui & Co has advised on some of Europe's biggest mergers and acquisitions this year, upstaging many blue-chip investment banks.