All you have to do is create a different folder on your web server... copy DW to it... and then install that instance of DW in that folder.
In such cases you'd be better off with a farm
, i.e. one DokuWiki installation would drive multiple wikis (with completely different sets of configuration, ACL, users, templates, plugins, etc).
Although I agree with Urfaust that namespaces sound like a better solution to the OP's question.