The Twitter Venn tool I published a couple of weeks ago didn't work properly in cases where the same word was used as part of multiple terms. For example, if you searched on 'toronto snow, montreal snow' it would always return empty result sets. I've fixed this problem so now you can do comparisons that include shared terms. This seems like a natural use case - you are basically asking how do these terms compare in the context of X. The example below comes from http://www.neoformix.com/Projects/TwitterVenn/view.php?q=toronto+snow,montreal+snow,vancouver+snow and shows how tweets for toronto, montreal, and vancouver compare in the context of 'snow'.
If you try it and still get empty sets you likely just need to shut your browser down and restart it.