Your first guess was correct: if a user with an iPad 3 has downloaded your 1024x768 version, and you later publish a 2048x1536 version, they will end up with two versions (the 1024 they downloaded and the 2048 version that's available for download).
And yes, your two renditions have to have the exact same information in every single field. This means not only the folio name and the other things you mentioned, but the meta for each article as well (the sidecar.xml thing is actually useful for that). It doesn't cause any issues; you just have to make sure that if you later make a change to one rendition, you have to do the exact same thing to the other.
I hope this helps