Even with their new pre-eminence, Tencent and Alibaba face some daunting challenges. : The technology world’s $400 billion-and-up club — long a group of exclusively American names like Apple, Google, Facebook, Microsoft and Amazon — needs to make room for two Chinese members. The Alibaba Group and Tencent Holdings, Chinese companies that dominate their home market, have rocketed this year to become global investor darlings. They are now among the world’s most highly valued public companies, each of them twice as valuable as tech stalwarts such as Intel, Cisco and IBM.