Also, I used the term "post 70s" so there's no confusion or arguments about what makes something "modern."

IQ: Subterranea

Swans - Children of God... most everything Swans really

I've only listened tо two the weeks old King Gizzard & The Lizard Wizard - Omnium Gatherum
-at bandcamp once, but it sure sounds like it' gonna and up a favorite.
Sufjan Stevens - Illinois

...unfortunately Guapo - Five Suns has ony been released on CD

(I also love the first and third Vektor-album who are both double, but it's progressive, blackened thrash metal, and may not be what you're looking for)

But how do you define "double album"?

In this day and age that becomes a bit tricky so either two cds or if it's issued as vinyl then two albums. If it was originally on vinyl then converted to one disc then that still counts as a double imo. 

Isn't Transatlantic's "whirlind" just one 60 minute track? I don't think I would consider that a double. Would you? If that were the case then a lot (maybe even half) of all recent prog albums would be considered double. I admit it's not always clear-cut though especially when a band like the Flower Kings has cds that are clearly double albums while others have an extra bonus disc. Should that count as a double? Ultimately, I'll let those who respond make that decision since there is an element of personal interpretation involved. 

The Flower Kings - Stardust We Are, Unfold the Future

Spock's Beard - Snow

IQ - Subterranea

Echolyn - s/t

There's precious few 1st Gen Prog 70's doubles deserving of the extended running time in my book. Even the excellent Lamb Lies Down on Broadway runs out of steam towards the end of disc two while Tales of Pornographic Lotions has only got two decent tracks. Frank Zappa's Lather, Civilization Phaze III and Uncle Meat all contain many fine moments but too few are sustained and the asinine dialogue between tracks is irritating. Works Volume One suffers from 20 minutes of diabetes inducing Greg Lake balladry while both Electric Ladyland and the White Album are very uneven and of course deemed 'Proto'. There's already been identified a clear problem in defining what exactly constitutes a 'double' in the digital realm. Double vinyl LPs lasted around 80 minutes tops and if the acknowledged masters of Prog couldn't fill those with sufficient top quality material what chance do the likes of Wobbler, Transatlantic, Flower Kings, Big Big Train, Black Bonzo et al have of filling an 80 minute single CD? Off the top of my balding head here's what's salvageable:

Out Of Focus - Four Letters Monday Afternoon

the Who - Quadrophenia

Jethro Tull - Living in the Past (is this a compilation of sorts?)

Btw, what in your opinion are the two decent tracks on Tales? 

Yes, Living In The Past was a compilation of mostly earlier JT tracks.

That'll teach me to skim the OP in future but it's because I'm old and smell of TCP.

Let's see if I can answer your subsequent question a bit more successfully:

I enjoy The Revealing Science of God and The Remembering but think the rest of 'Tales' just a giant hippy 'pot noodle'

When it comes to The Wall, I do prefer the first half overall, but some of my favourite tracks are on the second half, and again for me there is no filler, and not a single track I would drop, whereas other more favoured Floyd albums often have a track or two that I would be more inclined to “drop”.

People often talk about the drop in quality on the second half of Tales, but The Ancient is by far and away my favourite track on that lengthy album. And I don’t think I’ve ever heard a Yes album that I enjoy every track, but Tales probably comes closest - which is why it is my favourite Yes album.

It does become very difficult to determine what is a double album these days, just as (conversely) it came be very difficult to determine what is an ep. Ultimately, it comes down to what the artist decides to call it. I own plenty of releases that are considered eps that are longer in length than many of the releases I own that are considered albums. Anything over 20 mins seems to be fair game to be considered an album, and anything under 30 mins seems to be fair game to be considered an ep. That already gives a lot of crossover, without even taking into account some of the outliers (like Dream Theater’s ACOS “ep” which many, like Paul, would with good reason call an album).

In terms of double albums, length these days is simply not an indicator at all. You can find CDs that are almost 80 minutes that are NOT considered to be double albums, and albums of over 60 minutes that ARE considered to be double albums. In the latter case, it is generally very easy to see that the artist considers the album to be a double, because all the music would fit on a single disc, but they have made the effort to put it across two. These days it is not uncommon for the two discs of a double album to be given different (sub)titles, which makes it even more obvious that the album is to be considered a double. And, where for pragmatism and expediency a double album is released on a single disc, the (sub)titles of the two halves of the double album make it clearer than it is still to be considered a double, rather than a single album.

The most recent double album that has really resonated with me, is Cronofonía’s eponymous debut, which I know I have mentioned several times on this forum. It’s weird, too, because it’s not really in my wheelhouse, and not at all the sort of music I usually listen to and/or enjoy. But there is something about it that has me quite enamoured, and there’s not a track I would drop over it’s two disc length.

https://homerecordsbe.bandcamp.com/album/cronofon-a" rel="nofollow - https://homerecordsbe.bandcamp.com/album/cronofon-a

The first modern double album (if we exclude the Gn’R Use Your Illusion albums, which would have been a double if released as one, but instead they made a huge statement by releasing them simultaneously as individual albums) that really grabbed my attention was probably Smashing Pumpkins’ Mellon Collie. Of course neither Gn’R nor the Pumplins are prog, per se, but both those double albums contained prog even if the whole would not be considered so.

But although I would have liked to own Five Suns on LP, its obvious that is was never composed with a double album-release in mind. It doesn't really fit and [untitled] is one minute of silence (which sort of make sense on the CD, but not as the end of a nine minute lone album side):

1 1. 4:31
2 2. 10:19

3 3. 10:30
4 4. 12:57

C
5 5. 7:55
6 [untitled] 1:00

D
7 Mictlan 8:58
8 Topan 6:37

So yes I think double album is also about the artist or band's mindset, not just length.

wouldn't all TFK albums be a double LP by the shear jam-packing of a simple CD?

Already a single 3-mins TFK song feels like a 10-hour aural & physical torture