Thinking back about this whole moving between classes on aircraft my recollection is that on domestic flights the FAs will sometimes, maybe even usually, try to keep passengers from coach entering the business/first class section. I've seen just about every variation from aggressive blocking to paying no attention. Again just my recollection but it's only been on International flights where I recall an actual announcement that moving from class to class was prohibited. As a couple we've never flown separate classes on the same flight so that's never been an issue for us.