The Editor Interaction Analyzer is a tool that is useful for showing the overlap in pages edited by two or more editors. You simply add a username to each line, optionally select a beginning and end date for the search, and let 'er rip.
On the results page, you'll get a list of pages, the total number of edits for each, and the minimum number of days between edits by each. For example, here's the comparison between Wikipedia's famous co-founder Jimmy Wales and Wikipedia's almost-famous most-active editor, Justin Knapp.
Why would you want to do this? Most of the time you probably won't. Very few editors make use of it. Its use case is primarily limited to sleuthing out connections between editors who might actually be the same person, or are working in concert without saying so. Or, you know, it might just be fun.