Should all authors on a paper be comfortable explaining every aspect of the paper?
By most standards, authors of a scientific paper need to have made substantial intellectual contributions to the paper (e.g., coming up with the idea behind it, proving a required result, etc), made contributions to the write-up of the paper, and ultimately approve of the paper draft when it is submitted to a journal. As such, I think both you and your collaborator meet authorship criteria even though your grasp of topic Y is not as deep as your collaborator (and presumably your collaborator's grasp of topic X is not as deep as yours). At the very least, it is not dishonest to proceed as an author.

As for whether an author should be able to explain every aspect of a paper, I would think not necessarily. My field is very interdisciplinary, and it is not uncommon for clinicians to work with statisticians on papers. They are all experts in their respective fields, but their knowledge of the other person's field is almost guaranteed to be less than their collaborators. Yet, it is still possible for them to write papers together because their knowledge bases are complementary.