It took me 18 months to get the call for a 2nd interview. I pulled all the strings I had to try and get a 2nd interview when my 12 months was up, but at the time they said it was 18 months between interviews. Talking to Megan and Rebekah at Air Inc gave me a little insight into their process for 2nd timers. Apparently they plan a certain percentage of their interviewees to be 2nd timers. I believe she said it was 5% at the time. So, if they are calling 200 people to interview in a given month, then 10 of them will be 2nd timers. I think the number and qualifications of the pool of 2nd timers will determine when (if) you get a call for another interview. In addition, Megan confirmed that there is about a 75% success rate for 2nd timers.
In addition, a SWA F/O buddy was talking to a well-connected CA on a trip. Evidently your performance in the first interview can also affect whether you get another call. This passes the common sense check, as you would think they would want to call a guy who barely missed the cutoff on the first try versus a guy who they ranked at the bottom.
The bottom line is that none of us can do anything to affect their system. Get the job with Delta because there are no guarantees with SWA. They may never call you back, and even if they do you might get turned down again. Their hiring process is far from perfect. If I were in your shoes I would put all my effort into getting the job with Delta. Keep updating your SWA application and if they call then you'll have a good decision to make.