(Photo by Al Macias - KJZZ)

A US Airways plane at Phoenix Sky Harbor International Airport.

American Airlines and US Airways have started selling tickets for the other airline’s flights. It only applies to certain routes, including hub and international flights and some of their East Coast shuttles. They have also started allowing frequent fliers to earn partial miles when they fly on the other carrier.

Next month, the two airlines will both be located in Sky Harbor’s Terminal 4, and in March, US Airways will join One World, a coalition of international carriers which American is already a part of. And they're facing the continued challenge of pulling off the merger while keeping travelers happy.

Don’t let anybody tell you otherwise: creating the world’s largest airline is a tall order.

"There are literally hundreds of thousands of decisions that you have to make," said John McDonald, a spokesman for US Airways and American Airlines.

"Throughout all the industry, we’ve learned from our mistakes," McDonald said. "In those areas where we can make change quickly and it’s easy to do and we can minimize the disruption, that’s what we’re doing, but we’re also being thoughtful and deliberate about those kinds of things like computer systems and other systems that can really create some difficulty if we don’t do them just right."

McDonald said all customers are important, but acknowledged that business travelers make up a disproportionate share of revenue for airlines.

But they often do not get much more than lip service, according to Joe Brancatelli. He edits a website for business travelers, and said they value simple perks like extra legroom in economy.

"American Airlines has a product called 'Main Cabin Extra,' which is their premium economy product," Brancatelli said. "US Airways has nothing like it. How long is it going to take for Doug Parker and his crew to realize that this a product business travelers want and need?"

And Brancatelli thinks things like possible labor disputes down the road mean the merger will not be as smooth as the airlines have said.

"Synergy is one of those words you mark down like a political promise. They talk about while they campaign, it never happens," Brancatelli said. "I don’t see the combined American-US Airways being any stronger than the two carriers were operating separately."

And they will not just face the challenge of finding the promised billions in synergies. They will have to make some very specific policy decisions. McDonald mentioned one such instance from a past merger.

"One of the airlines allowed you, as a second piece of luggage, to carry antlers from an elk or a moose that you had shot at no extra charge, and the other airline charged that as an oversize piece of luggage," McDonald said.

And in case you were wondering, McDonald said the new airline does not have an antler policy just yet.

Updated 1/17/14 at 4:12p.m.