If I may comment from a non greek perspective......
I personally don't see anything wrong with it. They (fraternities and sororities) already have single events together....so having a whole week together was bound to happen.
It's really not that serious, at least not to me. There are bigger problems in Greekland IMHO that people should spend more time pondering over, instead of whether or not ABC and DEF fraternities are having a week together.
Just my 2 cents.
PP