That's where the Cohesion metric comes into play for OO (and maybe other programming paradigms?). Aka: is each class a single cohesive unit, or do you effectively have two (or more) classes melded together with disjoint data and methods.

Something I think is particularly interesting about this approach is that it is language agnostic. It's probably even independent of "programming". It could also be useful for general documents: if I edit wiki page A and always edit B too, should they be the same page instead?