This is a good question.
Let’s consider your first example.
Obviously, “neighbors” is plural. Thus, “make” is correct in the initial statement. But, in the response, “who” means “which one of the neighboring households.” And “which one” requires “makes” because “one” is not plural. “Neighbors” is a word that may refer to individuals considered separately or to households, which are considered collectively. The initial statement does not make clear which meaning is intended, and the response assumes, perhaps incorrectly, the collective meaning.
The second example has an initial statement referring to two people. Thus, it is plural and requires “spend.” The response, however, is very odd because it implies that the subject of the initial statement was not heard. Therefore, the natural response would be “who spends” because there is no basis for assuming a plural subject. If, however, the responder heard “both” but not the names, the natural way to clarify would be to say “which two spend.”
A different way to think about this that a dialogue involving ambiguity or misunderstanding may lead to contradictory grammar.