The car the Blue Brothers drove.

Ford XC Flacon

May I ask what size tube they use?

Yep, I think they're M-2s. Those are UNMISTAKEABLE! They could probably be mounted back to back, with a small arm (like the ones that attach a shoebox) holding each. Or they could be custom? IDK how they're mounted, but correct me if I'm wrong, i think they are M-2.

F20T12 have about the same intensity as F40T12, a little less (1050lm 2ft F20T12 vs 2500lm 4ft F40T12 for 765)

I am trying to understand why then bare F40T12 was cosidered too high intensity in the US but ok in Europe

And why over the years bare incandescent lamps, T8, T5HO, and now even bare LED chips without any diffuser (in decorative luminaires for living room no less) became considered ok. What changed ?

But the short F20 would have all the light concentrated into the short tube, so too bright.
The need is for lower intensity, larger surface light.
Yes, PMMA were used quite long, but if it needs to diffuse the light, it needs either some difraction pattern (the ridges then tend to hold the dust), or have some "milky paint", which tends to absorb a lot of light.
But if you look at common pricing, the F40T12 cost was about the same or frequently lower than the F20T12, the only reason being the F40 was manufactured and sold at way higher production volumes.
Originally the reduced intensity "residential" ballasts were designed to run the common F40 tubes, just at about 25W. Then dedicated 25W tubes appeared, with just thinner filament, to reduce the cathode heating power consumption when the cathode does not need to be that beefy for the reduced power. It is true the 25W version become a lower volume odd wattage tube (although sharing way more production tooling setup with the main highrunner than the F20T12), but the standard of using the lower power for residential applications was already set at the time the dedicated 25W version tube was introduced..

Being T12 the lamp is fairly diffused already, its intensity (when looking at the lamp) is not as high as T8-T5. Diffusers made of PMMA sheet formed to the wanted shape with vacuum were used in luminaires already in the 70s if not 60s

The F20T12 was already made, its not like the F25T12 could eliminate all demand to F20T12 ever

And a choke (of the size of a PL choke for us) would definitely cost less than autotransformer ballast

One of these beauties Citroën DS 21!

All seems so wild to me on the other side of the world....

Why would effort be made to produce dim, inefficient, big, and requiring expensive ballasts F25T12 for home use, and then put them in twin fittings anyway ?

when a single F40 does as good job ?

Or F20T12 at its normal power, half the size, with just a choke ballast, would put out the same light as one F25...

(And if it has a few Lumens less, then overdrive it by a couple Watts to get those Lumens up....)

The brightness of full power operation would be way too much for the domestic use, because how close these fixtures are.
Yes, it would be possible to use a diffuser with a single F40T12 tube, but that is either very expensive, prone to getting dirty or inefficient (wastes, absorbes, a lot of the light). Mainly in the past there were no suitable materials making a good, efficient long lasting diffuser for low enough price. When using more lamps and reducing the power density you get away with bare bulbs, so no cost for the diffuser, no light lisses in it, nothing to collect dust that is then blocking even more light, nothing to yellow down,...
Plus shorter tube costs more to make than the mainstream size made in huge quantities (because it needs retooling, not just replace the filament feed and the etch stamp and keep the rest of the machinery the same as with the mainstream F40T12), so also a cost benefit.
Plus two lamp ballasts cost about the same as a single lamp full power ballast, so no saving there.

Using two physically large lamps and feeding them by reduced power was just a cheaper way to get what was needed.

@Ash - Don’t worry, our residential single-lamp fixtures aren’t free from underpowered ballasts either. In fact, I think that’s where they started. The whole thing really is one of the biggest scams in lighting history. If the ballast underdrives the lamp by a few watts, fine. But under 60% power is ridiculous. And the companies still advertise the full lumen output on the lamps and not even mention that the fixtures are underpowered, so customers would buy these powerful-looking fluorescent fixtures expecting to get commercial brightness but would instead be met with regret and disappointment.