Does it makes sense for two domain servers to contain exactly the same set of names? Why or why not?
Yes. This is very advantageous for 2-domain servers having same set of names. If here is only one server than traffic upon one server would be intolerable, since this would be the only server to get the entire request and handle them suitably. If there is more than single server having similar set of data then geographically closet server will respond therefore reducing the load on one server. And if one server is down because of some problem then other server having same set of data can be utilized to fulfil the incoming requests.