I believe it came with the Note 2 release (though possibly not with some specific models, like the US ones). I got my (international) Note 2 a few weeks after it came out and, IIRC, I had a least a couple of multi-window Google apps (maybe Chrome and Maps but not Gmail and YouTube?) from day one.
From what I remember, the Note 10.1 (which came out a few months earlier) was much more limited on that front - only some Samsung apps supported multi-window, but that seemed to help pave the way for the Note 2 (and improved support on the 10.1, of course).