Need to filter dimension where two attributes are the same
I got it!

Basically the filter expression was correct, but I needed to Crossjoin the set what I wanted to filter against...

FILTER (
  CROSSJOIN([Student].[Major].[Major], [Student].[Minor].[Minor]),
  [Student].[Major].Properties("Key") = [Student].[Minor].Properties("Key")
)


I know I answered this quick but I've spent the best part of 10 hours on this today :P