Nearly 20 months after completing their $3.2 billion merger, United and Continental are grappling with the messy business of stitching together two sprawling operations, the Wall Street Journal reported today. The process has left many longtime customers fuming, bewildered many of the company's agents and slowed revenue growth. The company can ill afford to alienate its elite frequent fliers, a notoriously finicky bunch who make up about 1 percent of frequent-flier membership but account for about 25 percent of airline revenues. Many of them are business travelers who fly constantly and pay top dollar for their tickets. A decade of airline consolidation has proved the business logic of such unions, with the industry broadly profitable once again. Another such deal may be on the way as US Airways Group Inc. is pursuing a merger with American Airlines parent AMR Corp., which hopes to remain independent when it exits bankruptcy protection.