OP - Honestly, it is hard to give you an idea of how likely it would be to get two seats together without paying for them. It all depends on how full your flight is, how many people have paid for their seats, and how close to the 24 hour mark you check in. What you can do, is continue to monitor the flight and seats available for check-in. If there continues to be many seats available AND you can check in right at 24 hours, you stand a good chance of getting two seats together (though, there are no guarantees). If the number of available seats starts to dwindle, you may want to pay to pre-select your seats.
That, in the best nutshell, is the case of the matter with selecting your seats or not for airtran!
A longer look: It all depends on how full your flight is and if you have preferences where you sit (i.e. due you want to be in the front of the plane or do not want to be located next to the double lavs and no window row of 31 where 31d actually sticks out into the aisle and can get real friendly with anyone waiting to use the restroom, then pay for your seat assignment).
If the flight is full, Airtran may, no gurantees proactively seat parties together before the flights open for 24 hour checkin to not have issues of people being split up, but it all comes down to how many open seats there are (think of the airplane diagram for, say, a 717, there are 5 seats in each row, ac aisle def, in one row someone has paid for a seat assignment in 10d, another 10f, another 11a and c, and another 11f. Now comes a party of five if those are the last seats available then they would need to be split into 10ac, 10e, and 11de multiply this selected seat scenario by the whole plane as people want window and aisle seats and you can have a very hard time getting parties together plain and simple).

