"Being associated" (grouped together on the booking page) is not the same as "being co-terminals." PBI and FLL were never (IME) grouped together on the booking page despite being officially designated co-terminals. From the relevant (but now quite outdated)
portion of the FAQ:
PBI, a co-terminal for FLL, however, is not listed in the peculiar [Miami Area] "grouping", which contains just the single airport FLL.
Distance is not a reliable indicator as to co-terminal designation. Again from the FAQ:
Note: SAT-AUS and SAN-SNA are not co-terminals, even though the distance between co-terminals PVD and MHT is about 25% greater than the distance between SAT and AUS, and about 10% greater than the distance between SAN and SNA. Similarly, TPA and MCO, which are marginally closer than PVD and MHT also are not co-terminals.
That note was written before MKE service began, so I don't know whether WN considers MKE and MDW as co-terminals.
Southwest.com used to specifically list the official co-terminal designations, but apparently no longer does so—despite the CoC containing reference to "carrier-recognized co-terminal." Without that information on a web page, there isn't a good, easy way to determine which city pairs are co-terminals. Perhaps a CS&S agent can access that information on their terminal, or perhaps one of the SWA agents can provide a current list. Of course the list might change again as integration progresses. (I'm not familiar enough with the list of AirTran cities to know of any future co-terminal possibilities.)
The "self-service" way to figure out co-terminals is to use a promo code or sale that requires RT booking and see whether an A-B-C booking treats cities "A" and "C" as co-terminals. The risk in that approach is that the IT department may not be in synch with what WN "officially" considers co-terminal pairs.
Thanks for bringing up the issue; we do need an official source of information to update the info in the FAQ.