By Maria Ward-Brennan The most highly anticipated merger in the legal sector formally completes today, operating under a new name A&O Shearman. Magic circle firm Allen & Overy (A&O) and US firm Shearman & Sterling shocked the legal sector last May when they announced the proposed merger. With the merger completing today, A&O Shearman becomes one of the biggest law firms in the world, with $3.5bn in revenue and nearly 4,000 lawyers globally. Why?The aim behind the merger was to give both firms access to more markets. The combined firm now has 48 offices in 29 countries. Legacy firm A&O was part…