Southwest is taking the next big step in its acquisition of AirTran. Passengers can now book dozens of routes using flights on both airlines while using a single itinerary. This marriage of routes is a major step toward folding AirTran into Southwest's current route network. Southwest announced soon passengers will be able to book flights on any of the airlines' combined 97 destinations, including international, in one transaction. This is a big change for loyal Southwest passengers who have not been able to book international tickets on Southwest in the past.