Going into academia if the research is interdisciplinary If one is doing a PhD in an area that is on the borderline of a few areas (see the specifics below), would it be harder for them to find a job in academia after graduating?

To be more specific, the fields of linguistics (formal semantics), math (natural logic), and philosophy of language are closely connected. There is even a Dutch term which stands for the joint study of the three (which I cannot remember). Assume that one's PhD dissertation would be in that area(s), and the PhD itself would be a "dual" (double major) PhD in math and linguistics.

On the one hand, such a background suggests that more academic positions may be available (in applied math departments, or in linguistics departments, or in philosophy departments). On the other hand, being between the fields may be a disadvantage because the person doesn't actually belong to one specific field.

And a related question: what should that person do to make the process of finding postdoc positions easier?
We can't really make general statements about one focus of research being better than another. Maybe in this hiring season there are 6 jobs out there perfectly suited to someone doing X, while there are 7 that specifically want (or just prefer) someone doing X+Y. And another year the ranking of numbers might switch. 

Certainly "interdisciplinary research" has a strong positive connotation. My guess is that is because it most commonly entails the combination of a traditional (perhaps stagnant) old field with some sexy new field. The idea is that you are progressing toward "the future" where the field will be evolving in these new directions. While the old pigeonholing of research into particular categories becomes increasingly outdated.

On the other hand if you just combine a couple overcrowded fields that have been around forever you may not gain anything.

One possible drawback I can think of would be in teaching. Even when the hiring committee looks for someone with a lot of breadth, they still may require a lot of depth in their main field. They don't want to be stuck as the only one who can teach that boring old core required course forever.