According to the SDE site, this new package contains the following:
It features a vinyl LP, four CDs and a blu-ray audio and offers a new Dolby Atmos Mix of the 1971 album from Steven Wilson. His previous 5.1 mix and a hi-res stereo mix are also included on the blu-ray (but not the original mix of the album). On CD are two previously unreleased concerts, the instrumentals and new stereo mix (from 2014), 20 tracks of rarities and the original mix of the album (which is also on the vinyl in this set).
(Steven) Wilson shares his insights into the new mixes in the set’s sleeve notes: “For the Atmos Mix, I used my 5.1 as a starting point but found myself taking quite a different approach on some parts of the music, partly by virtue of now having the extra speakers to place the sounds in, but also because I’ve learned a few things in the nearly ten years since I first remixed the album…
So no, there aren't two 5.1 mixes - just the 2014 5.1 mix and a new Atmos mix based on the older 2014 5.1 mix. The 2014 CD/BD release is still available for much less than this new set, but if you really want the Atmos mix, vinyl and think all the bonus tracks are must have, then this new set is what you want. For me, since I already own the 2014 CD/BD set, the only thing I'm interested in is the new Atmos mix. I might be persuaded that the two live concerts plus the Atmos make it worth the high asking price but I'm waiting for reviews. Unless the performances and CD recording quality are both absolutely stellar (doubtful), I'm just not that interested in more live Yes.
That's not how I interpret what SW said. Taking his 2014 mix as a starting point doesn't mean the 5.1 auto-downmix of his Atmos mix -- the only 'new' 5.1 available here -- will be the same as the 2014.