Number of potential PhD advisors at a school
It sounds like this subarea might be somewhat marginalized in the larger area the department represents, and this is consistent across universities. Why is that ? is the subarea not particularly popular, or interesting, or does it lack the ability to get funding ? In all these case, you do have to ask yourself whether the risk of getting into such an area is worth it.

But on the more direct question of whether you should favor the few schools that have more than one professor, the right question is: what are you planning to do after a Ph.D ? And in what way can this professor (or professors) help with that goal ? If you want to go into academia (say), are these professors the dominant players in the area ? and so on.