Does it makes sense for two domain name servers to contain exactly the same set of names? Why?
Yes. This is very advantageous for two domain servers having similar set of names. If there is only one server so traffic on one server would be in tolerable, since this would be the only server to receive the entire request and handle them suitably. If there is more than one server having same set of data then geographically closet server will respond therefore reducing the load on one server. If one server is down because of some problem then other server having same set of data can be used to fulfil the incoming requests.