There are several cases where tags are synonymous, except for a hyphen. Apart from experts-exchange (which I doubt has a expertsexchange or expert-sex-change synonym), the tags should be automatically merged.
Examples include thread-pool versus threadpool and multi-dimensional versus multidimensional.
As to which one should be canonical: I'm more pedantic about English than most people, and even I don't care.
Edit: Related questions: Merge the tag 'httpserver' into the tag 'http-server' and http://meta.stackexchange.com/questions/89448/merge-web-scraping-and-webscraping-tags and Merge or ban howto and how-to tags and Make [wcf-service], [wcfservice] synonym to [wcf] and Tag Synonym Request: [asio]->[boost-asio] and Tag merge request: [feed], [feeds], [syndication-feed], [syndicationfeed]