I submitted 1 Interest to sponsor for a family size of 5 (myself, wife, mom, dad and grandmother). On the invitation letter itself it was specifically mentioned to send separate applications if applying for both parents and grandparents (page 2 of the invitation letter, 4th bullet point)
And I confirmed this specifically with CIC over the phone as well because it was pretty much repeat of forms for ourselves (sponsor).
I was expecting the same thing, both applications with same sponsor/invitation being processed together side by side.