(Reuters) – Wal-Mart Stores Inc (WMT.N) is losing two top e-commerce executives at a time the world's largest retailer is trying to step up online competition with the likes of Amazon.com Inc (AMZN.O). Wal-Mart said on Tuesday that Vice Chairman Eduardo Castro-Wright, head of the e-commerce and sourcing business at Wal-Mart, will retire on July 1 to spend more time with his family. In separate news, Nestle (NESN.VX) named Wal-Mart's executive vice president of global e-commerce for emerging markets as its new chief financial officer. Wang Ling Martello, who has also worked with Kraft Foods Inc (KFT.N), will join Nestle on April 1. The departures comes some six weeks after Wal-Mart said it was shaking up its e-commerce structure as it aims to build up its online business. Wal-Mart has had...