there's a lot more to say about a question like this than can be said in one post.
auburn-opelika and columbus-phenix city have very much the same sort of problem (although it's more profound with auburn-opelika) - there is still a LOT of discontinuity between each pair of cities. auburn and opelika do not feel like a continuous city; nor do columbus and phenix city (though the presence of the chattahoochee as a natural divider plays a role). for all the development going on in auburn, VERY little of it is taking place along the existing corridor of disgusting, blighted, forsaken commercial space that connects the two towns. most of it (even that which lies between auburn and opelika) is happening on previously undeveloped land. the density of the auburn-opelika metro would have to reach a critical mass before growth in any direction outside the two cities would make sense (not to say that sense will be the guiding factor in these cities' growth).
too, the 25 or so miles which separate phenix city from opelika is a pretty long, rural, and sometimes topographically challenging 25 miles. it would take a freakish surge of sustained development to urbanize (or even suburbanize) that corridor within the next decade or so. if these areas are to eventually merge, i hope it's slow growth all the way. the two areas are distinct enough one from another that they need a long history of shared needs and trends before they can see a meaningful and productive merger.
as trivia, columbus is virtually tied with augusta as the second largest city in georgia in terms of population. augusta (last time i checked the back of my trusty atlas) has approximately 5,000 more people than columbus. it will be interesting to see how columbus cotinues to handle its growth across the river and into alabama.
