Okay, I'm very much a neo, and I thought this was common behavior. We've had a "Diamonds & Pearls" party with the Kappas, Deltas & Alphas commonly have a "Jungle Jam" (elephants and apes), the SGRhos and Sigmas had a "Blue Beatdown" or something like that. And of course we've had an "Ivies & Ice" formal with frat.
Now to my knowledge, with the exception of the traditional connections, these events are planned because of scheduling conflicts. Each chapter has 2 NPHC delegates. Say AKAs want a party on this coming Friday and the Sigmas do too, it's easier (and more profitable) to just throw it together. We have a pretty good relationship with all the fraternities on campus, so it's never a big deal. And our relationship with the other sororities is decent as well, so it's no big thing if the Deltas and Alphas have a party together.. we'll still be there in full effect.
Now a whole WEEK together??? That's a whole 'nother story. I ain't NEVA heard of/seen that and I think it's ridiculous. When Skee-Week comes around, that's OUR time, and I'll be doggone if we're about to share it with somebody else!
We might have a co-sponsored event (for instance, church on Sunday with the Deltas or SGRhos, etc), but never EVERY SINGLE DAY. I think that's a bit much.