Past advice suggests that potential PhD students should be wary of attending schools with only one professor in their subfield. If you have problems with your advisor and want to switch, having that option would be invaluable.
But what if your subfield is not represented that well in the vast majority of departments? Almost all schools I'm considering applying for only have one professor in my intended research area. I'm a MS student at the moment and I'm sure what general subfield I'd like to do my PhD research in.
Should I just accept that this is how things are and try to check whether a professor and I would be good matches before accepting? Or should this make the few schools with more than one professor in this subfield more attractive? To be clear, every school I will apply to will have top researchers in this subfield. The comparison is more between a school with one top researcher and one newer researcher and another school with one top researcher and no one else in the subfield. The majority of schools are in the latter group.
I'm tempted to think that hiring committees avoid hiring additional faculty in my subfield if they already have one professor doing research in the area. They have their token professor and that's enough for them. Never mind that the professor's personality, perspective, and research methods matter!
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.