Here's an article from the Ft. Worth Star Telegram from 3 years ago about the ones Southwest is using.
In addition to the dual jetbridge at DAL, there is one at AUS, and ALB has either gotten or is in the process of getting 2.
I don't have anything to back this up, but I seem to remember reading that Southwest wanted to experiment with this because of BUR. BUR is the only Southwest station to not use jetbridges. Passengers board and deplane using airstairs at the front and back of the aircraft and apparently BUR had some of the fastest turn times in Southwest's system.