This has been brought up a million times and there has never really been a definitive solution. We desperately need some more options here! Ideally we could do something like...
+If Instance (with value of x) collides with Instance (with value of y)
-Pick Instance (with value of x) and do this
-Pick Instance (with value of y) and do that
You might be able to pull it off with functions or something but eh...good luck.