shaman monk makes an outstanding duo, as monk can tank and pull singles and deal pretty good dps while shaman can slow and heal and buff. Cleric and tank(warrior, sk or paladin) or cleric and enchanter is a great start to a group if you hope to start up groups together. You can find things to do with most any 2 classes. Generally the advice is the same to all of these questions and that is play the class that appeals most to you and you will have more fun even if the duo is not optimized.
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