Keep in mind that there are two different ways to have a "second area". One is "X and Y separately" in which there is no necessary connection between X and Y. And it is often what smaller departments are looking for--each faculty member has to cover more than one area, and as long as all the desired areas get covered, it doesn't really matter how they are divided among the faculty, though of course there are more or less typical combinations. Having taken a single graduate course in an area may or may not be enough to convince a department that you can teach an undergrad course in that area. Having TA'd the corresponding undergrad course is more convincing, but barring that, having done some original research is the usual proof-of-qualifications to teach an area.
The other way is "X and Y together" in which your research somehow draws on both areas, in an integrated approach. With this kind of research program you might find that you actually feel kind of narrow in your specialization ("I do X, but (only) the kind of X from a Y perspective"), but you will seem broader than someone who does X without the Y, and you can certainly apply for jobs looking for X and Y. We think it is often the case that larger departments look for this. For instance, they may already have an X and a Y, and now they want someone to "build bridges" (a common buzz-word) for both the faculty and the students.
So our advice to you for right now is to keep in mind this goal of finding a secondary area when you select any further courses (that is, in addition to courses that you feel will help you focus your primary interest). You could then talk to the relevant faculty about how these areas might be integrated in your future research program, if that seems desirable to you.
More-general advice is to, some day, peruse the job listings on the linguist list (http://www.linguistlist.org/jobsindex.html). Look for jobs advertising for a specialist in your area X which you could imagine yourself applying for, and see what else, if anything, is typically desirable in an X. Good luck!