On Heidegger's "The origin of the work of art" and aesthetics The "in-between" refers to the collective group of a historical people and, of course, the artist dissolves in his/her work of art. So it' s really hard to identify which person Heidegger is making this claim for, it probably goes to communalities that share the same historical world (it certainly doesn't refer to subjective experience).
Also, *unpopular opinion*: I haven't found anything Nazi-related during my reading of Heidegger, except for this mentioned above. This grouped "return to Being" through art looks kind of similar to the "Nazi political regime as a work of art". I'd really like to hear your opinion on this -controversial, I know- claim.