ATA Airlines and Southwest Airlines have added another dimension to their working relationship.

As part of their existing passenger sharing deal, travelers on ATA’s Dallas/Fort Worth to Chicago Midway service can now buy Southwest tickets onboard.

Southwest will also soon start to sell tickets for international routes operated by ATA.

Details of the international routes to be opened to Southwest customers have not yet been released and ATA’s current international program only features flights to Mexico.

One ATA destination that is proving popular with Southwest customers is Hawaii.

According to Gary Kelly, Southwest CEO, the popularity of ATA’s Hawaii flights is currently causing a barrier to integrating the two airlines’ frequent flyer programs.

Kelly explained that the demand created by allowing Southwest customers to redeem their credits on the Hawaii service could well “overwhelm” ATA.

The close relationship between the two companies began in 2004, when Southwest lent financial support to the then-struggling ATA Airlines.