There is no way to have different internal scale on each screen. The core displays both at the same time and we just chop the screen up and organize it.
I’m not even sure it would be possible, even if the core devs wanted to.
You’ll just have to find a good compromise. Either 2x on both, no scanlines at all, or live with the first shot. It all depends on your priorities.
What about reducing scanlines on the bottom screen? Is it possible to tweak?
Nope. You could try using a local override by manually editing the preset file. It might look better using easymode or guest-mini as the base preset.
Read the instructions near the beginning of the file carefully.
That’s sad :c
Also, I found strange bug with vertical preset For some reason there are pretty big yellowish parts on both screens (On lower one it’s a vertical line in the center of the screen). It persists no matter what scale I’m choosing, nor changing texture filter type
That is not a bug. It is a side effect of not using integer scale.
My guess is that you are using increased internal resolution scale.
While this is possible, I don’t do it, and I don’t recommend it. It can cause issues like this in any and all screen shaders.
The solution is to NOT use scanlines or a mask… or if you are using the LCD-GRID, to use the pass-through base preset.
Just wanted to share that I have published the initial release of my koko-aio presets.
The project thread is at…
https://forums.libretro.com/t/duimon-koko-aio-presets/45136
Woo-hoo!!! 
@Duimon just a quick question: is there a way to fill the whole screen with your Duimon-Mega-Bezel-Potato shaders on an Android device? On my PC this is what I get on my 21:9 - 3440x1440 monitor, the whole screen is filled:
on the other hand this is what I get on my Android device (19.5:9 - 2340x1080), black bars on both sides:
No easy way. It would require the use of a PC that had the same screen resolution and some image editing. I could do it with just an image editor and some tools in the Mega Bezel, but phone screens vary and I can’t justify creating multiple sets.
I do have some overlays with carbon fiber that are designed to fill that black space and be used in conjunction with the shader.
That automatic screen fill stuff is essentially a AAA feature and needs some horsepower.
Understood, thank you.
Just to clarify. I would consider doing a subset of the Potato presets at 21:9. Here is a shot from a Potato preset I put together in about 2 minutes.
It is 21:9 and I used the “Windowed Mode” fixed window size option in Retroarch to get the image, along with the built-in masking tools in the Mega Bezel.
The image is around a 1 MB. A night image would be larger. The only caveat is that RA saves the image at 3438x1438 so there is a 1 pixel black line around all 4 sides where I increased the canvas size to compensate. (It might work just as well with no canvas adjustment.)
It would only take a few hours to set up a small collection of Potato presets. I’m thinking all the major CRT screen systems and the main handhelds. (Excluding the GBA and Lynx until I update them in the near future.) So a small set of ~20 presets.
I would create a separate repo for the collection to keep things tidy.
The issue with doing something for cell phones is the lack of a screen size standard. From what I could gather, phones that would run the Potato well would have one of three aspects.
19:9, 19.5:9, or 20:9.
And I have no idea what the actual screen height and width are to set the RA full screen resolution.
If things go easy enough with the 21:9 I would consider doing the others if I could get the needed info.
So I did some digging and phones are weird.
The iPhone X and the Google Pixel 3 XL both have 19:9 aspect ratio. The resolutions are 2436x1125 and 2960x1440 respectively. Neither work out to 19:9 so even though the aspect is the same the resolution is independent.
So… doing some math.
3610x1710 is a perfect 19:9.
3822x1764 is a perfect 19.5:9.
3840x1728 is a perfect 20:9.
All of which can be done on my 4K monitor using the above method.
I have no idea whatsoever if they would work as expected on various phones with those ratios.
Here is the article with the aspect ratios. If I can find some members of the community to test quick sample presets, I would put in the work.
https://www.aiseesoft.com/resource/phone-aspect-ratio-screen-resolution.html
There are at least two phones listed for each of these aspects.
I’ll even take some lists of requested systems, possibly expanding the number of presets by a bit to cover the desires of those willing to give input.
How interested is everyone?
I’ve decided I will do a single complete set of 21:9. That should look better on all these devices that the 16:9 we have now.
I will follow my current potato model using 2560x1080 as the resolution and put them in a “Duimon-Mega-Bezel-Potato-21x9” repo.
First I am going to work on redoing any handhelds that are clipped at the screen edges, so they can be scaled and will have something to show in ultrawide.
That’s amazing! Happy to test the set on my Samsung S23+ (1080 x 2340 pixels, 19.5:9 ratio).
It would be great to see what the 21:9 looks like on that screen.
I have a test preset uploaded to a repo here.
Click on each file and then the “Download Raw” button. They need to be in your root “shaders” folder and you need the Potato presets installed in the “Mega_Bezel_Packs” folder.
I left the image at the size RA generates with a screenshot so it is losing 1 pixel on all four sides. This will make the screen relatively 1 pixel smaller, but we’ll see how everyone feels before I go through hundreds of screenshots to expand the canvas size. (Although I could probably do it in a single pass with smart objects and a script. 
)
This is how it looks on my phone:
It’s a bit cropped on the logo and buttons. By the way will you make the bigger monitor bezel version too? This one:
I expected that but it’s better than how it looks with the 16:9.
Yes. Since this is a resolution that is common for ultrawide PC monitors, I will do a complete set. (e.g. It will contain every preset my current Potato pack has.)
I am still willing to do a subset for the other resolutions. All it will take is a community member with a phone using one of the resolutions, willing to test it, and I will do a subset for that resolution.
Since you so graciously accepted my invitation, there is now a folder here with a 19.5x9 test using the larger screen preset.
In this test I edited the retroarch config (Knowing it was going to clip the screenshot.) to be 3824x1766 and ended up with a perfect 3822x1764 image.
Please test it for me when you can.
Fantastic! It’s perfect now. Just one thing: the game doesn’t fill the screen whereas the potato 16:9 fills it (just tested it).
Edit: I figured it out why: you used the wrong slang reference in SNES_19.5x9.slangp. The right one is: SNES-[POTATO]-[Guest-Mini]-[Custom-Bezel_001].slangp
Did I really? 
Could we see a shot of it with the corrected reference?
Is it possible to use Duimon bezels with mega_bezel/Variations/Smoothed/SMOOTH-ADV_3_Extra-Smooth.slanggp? Would I just copy all 49 parameters in the settings?
