US Airways and American Airlines are taking the first steps to combine their flight networks, with a codeshare agreement between the two companies set to take effect Jan. 23.

The new agreement will allow customers to book travel on either airline’s website.

“Through this codeshare agreement, each airline will sell tickets operated by the other carrier using its own code and flight number, providing customers easy access to the combined company’s network,” US Airways spokeswoman Michelle Mohr said in an email.

The codeshare will start with select flights and gradually expand. In Charlotte, N.C., the airlines said, US Airways-operated flights to American’s hubs in Dallas/Fort Worth, Chicago, Los Angeles, Miami and New York will have American flight codes as well. American flights from Charlotte to other hubs will also receive US Airways codes. US Airways’ East Coast shuttle service between Boston, New York and Washington, D.C., will also start carrying American flight codes.

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US Airways and American completed their $17.8 billion merger in December. But they must continue to operate as separate airlines, keeping their individual names, until they receive a unified operating certificate from the Federal Aviation Administration. That’s expected to take 18 to 24 months.