So they're totally unrelated documents, and you want a list of every word that appears in both?

IMO, strip the punctuation(find and replace to turn periods into nothing, commas into nothing, etc.), copy it into Excel, tell it to treat it as space-delimited text(so that every word will be in a separate cell) with document 1 in sheet 1 and document 2 in sheet 2. I'm assuming the text will all be vertical, so from A1 to A9999(or whatever). Then put the following formula in cell B1 of worksheet 1, and copy it down to the bottom: =IF(ISERROR(VLOOKUP(A1, 'Sheet2'!$A$1:$A$9999, 1, FALSE), "", A1))

Column B will then be a list of every word that appears in both. (To explain the Excel a bit, the VLOOKUP finds the text of that word on the second sheet, and the ISERROR strips out the errors you'll get when it doesn't find them, leaving only the text of the duplicates)

Alternately, there may be a Web utility that'll do it more easily.