what frats and sororities party with eachother?

If you're concerned about who's partying with who as a rush you have your priorities all mixed up. All organizations pretty much have scheduled parties with each other at some point in the semester. To say that certain organizations go to certain places for afters just isn't true. Here most of the time people go out to houses with their friend group, regardless if their in the same organization. And also the party itself matters, if it's lame you move on to the next one and so on. Obviously some people and organizations like to stick together but it's more because they are actually friends with one another not because of the "organization" they're in. There are many close brother sister organizations who have members that don't enjoy partying with their respective pairings. Bottom line don't rush based on parties you will be disappointed.