First I would suggest you consider this carefully.
The Airtran Companion Pass costs 100 credits, those credits are gone forever.
The pass is good for one year.
Whether you will get use out of it is a good question and depends on how often your really go. With that many credits and points you can fly to CUN 8 times in business class or 2 round trips together, 4 if you can get A+ tickets, subject to limited availability.
At present you cannot use an Airtran CP on Southwest or the other way around.
Keep in mind that Airtran is supposed to be gone by this time next year. Southwest is transitioning Airtran flights to Southwest. No one really knows when CUN will go but it seems likely 3Q of this year.