Last week saw two big stories in the airline business — stories that were the focus of industry comments and developments and might affect your future travel. 1. Frontier-Spirit Merger Two of the country’s largest low-fare lines announced a merger: Frontier will acquire Spirit, pending government approval. The combined airline will still be small compared to the four giants — it will account for just about 7 percent of the total domestic market — but it will be substantially larger than Alaska, JetBlue, Allegiant, Hawaiian, or Sun Country. Why it Will Work. Industry folks see this as an easy combination to put together: Both lines share very similar business models and both use only Airbus 321-family planes. Top management and ownership are familiar with both lines’ operations and systems. Even though the lines compete at lots of individual airports, they overlap on only about 40 percent of their point-to-point routes, …