After beginning in an experimental phase, passengers of Southwest and its subsidiary, AirTran, can finally book connecting flight as one ticket.

The development of the combined airline is a big step for Southwest, but it did take nearly two years to complete and caused many customers hoping to fly both airlines the stress and inconvenience of having to buy two separate tickets.

"We began rolling out shared itineraries in January in a handful of markets, and we've gradually ramped up the initiative where we now connect our entire network across both Southwest and AirTran," Bob Jordan, Southwest's chief commercial and president of AirTran, says in a release. "With a connected network, we can offer Customers more itineraries, more destinations, more low fares, and a taste of what's to come once the integration is complete," according to USA Today.

The linking of the two carriers allows Southwest to cover more of the United States' map. USA Today, gives the example of "cities such as Richmond and Memphis still having service only on AirTran. So, prior to the linking of the carriers' schedules, customers from those cities could only connect to Southwest flights by buying two separate itineraries. Now, however, they can buy a single-ticket itinerary that includes and AirTran flight to Atlanta and a connecting Southwest flight to a destination elsewhere."

This also gives Southwest the ability to offer international destinations, including Aruba, the Bahamas, Bermuda, the Dominican Republic, Jamaica and Mexico that are currently only available through AirTran.

Southwest still needs to make upgrades to its booking system, but eventually, it will allow the airline to sell international itineraries.

Southwest calls its "transition to a single ticketing system" a "a large and complex process that is expected to be completed by the end of 2014." Some industry observers, however, have speculated that could slide into 2015, according to USA Today.