A chapter cannot have two disparate elements. It can have two sections, if the point is to compare the two things head to head or to a third thing, but it needs to have either one central focus, or at least three. You can have one-act plays, or three to five; or string quartets with any number of movements, but not two. A painting can have three panels or one, but two is more awkward. (There are exceptions. Don't be giving me 100 examples of musical pieces with two movements. I know they also exist but I feel they aren't as typical.)
The reason is that two elements seem to demand two separate chapters, one for each element. So I could have a chapter on Lope, Tirso, and Calderón, or a chapter on just Lope, but not one on Lope and Tirso.