I fully admit that this isn’t very intuitive (we’ll have to figure out a better way), but it should still work like this: if you want to break up the quote in the middle, place the cursor at the end of the last line of what is supposed to be the first quote, then press enter to create a new empty line (still part of the quote); now press enter two more times, this should split the block quote and create a new paragraph in between. We ought to do better than this, but it should work this way.