I was reading Does Ahch-To actually have two suns? and it seemed to me like it'd be very easy to get the impression that the twin sunset was all in Luke's mind since I don't remember seeing two suns on Ahch-To before that scene.
So my question is, besides that final scene in The Last Jedi, do we ever see the two suns onscreen in the film?
No. In this comicbook.com interview with director Rian Johnson, he states:
We've never seen two suns over the island before. And it's the only shot in the whole island where you see two suns and it's slightly ambiguous as to whether he's actually seeing it.
As far as (heavily based on the movie) Fry novelization goes, we do.
The first mention of Twin suns is in the prologue, discussing Tatooine
The second mention is Chapter 7
The late-afternoon suns hung low over the islands of Ahch-To, lengthening the shadows of the old stone huts. Below, the surf sighed, a rhythmic sound like static. Luke Skywalker sat on a bench outside his simple dwelling, next to a morose Chewbacca. Rey hovered nearby, reluctant to interrupt the two old friends in their mourning.
This is confirmed to not be a typo in Chapter 9:
He’d spoken! She decided to count that as progress, and smiled as she followed him back down the well-worn path, trailing him until they reached the tumbled rocks and narrow beaches that fringed the shoreline. Seabirds called overhead and the salt air was sharp in Rey’s nose. On the beach, sea sows lounged torpidly in tide pools, waiting for the suns to warm them.
and Chapter 13
Dawn on Ahch-To found the island shrouded in mist, tinted a fiery crimson by the rising suns.
and, when Rey is feeling the Force:
She could feel the warmth of the suns — not just on her face but on the rocks and the surface of the ceaseless tumble of the water. And cold, too, which surrounded the dark places where the roots of the island and the seafloor were revealed as one and the same. There was peace — mother porgs with their eggs, sheltered and safe in warm hollows—but also violence that left behind broken nests and shattered shells.
and Chapter 17
The suns were dipping toward the horizon as Rey and Luke entered the Jedi temple, facing each other across the font in the center of the ancient space.
and Chapter 35 (when Luke is doing his battle meditation while projecting to Crait)
On Ahch-To, the suns were setting, bathing the peak of the mountain housing the Jedi temple in luminous orange.
The last mention (searching for "twin", "dual", "suns") is in Chapter 35, where Luke fades away - the one that matches the movie scene:
Luke opened his eyes and fell onto the ledge, the pebbles plunking down around him. He lay on his back, his breathing ragged with exhaustion. The twin suns had touched the horizon and were sinking into the ocean.
BTW, the wording here isn't ambiguous, he does see twin suns.
As an extra bonus, Luke's last thoughts in Marvel's "The Last Jedi" comic book are:
And so it ends as it began. By the light of two suns
Yes. You see two suns over Ahch-To earlier in the film, but not together.
Rey finishes practicing with her lightsaber, and we pan across to the sun setting behind a mountain range.
From a slightly different angle (but evidently at the same time as the scene above), we see a different sun higher in the sky and no mountain in evidence below it.
