this is the closest I have gotten:
I had to change these three settings, but it’s not perfect (and it’s worse when using the vertical preset where the screens have the same layout as the real 3DS. I’m pretty much unable to set the correct aspect ratio for the second screen):
I’ll have to give it a try myself when I have the time. Any advice on parameters until then would only be a guess.
I may find some time tonight or tomorrow night. 
If you want a guess, I would try 16:9 aspect on the first screen and “Same as 1st Screen” on the second. (Cropping takes care of the edges to make it appear a different aspect.)
as you can see in my previous post, these are the exact settings I used (top screen 16:9, bottom screen same as top). But this still results in both screens being too large and cut off (see screenshot). Also the bottom screen is out of proportion, but this is less visible with this preset.
Here is a screensot with a different 3DS preset, where you ca see the problem better. As you can see, even with 16:9, the top screen is still a bit too big, and the bottom screen is way too large.
You can scale and position each screen independently, but I suspect it will take more tweaking than that. (I suspect that “Same as 1st” negates that. Oops. 
)
I am a bit confused as to why this doesn’t work OTB.
I am quite busy ATM but I will try to get some time to test it myself.
Edit: It just occurred to me that PAR is probably the correct setting for the second screen, except that the shader get’s that info from Retroarch so it isn’t available here.
Given this, although for now I’m thinking it is still possible, I wouldn’t be surprised if dual screen presets with different aspect screens just doesn’t work.
This is the correct answer.
The issue is with PAR being one of only two options. It appears that Shaderglass treats PAR as the full width of the screen.
Using 1.67 as the aspect ratio for the top screen, and “Same as 1st” for the second… then scaling the second… we can get this.
But it isn’t correct… the second screen should be 4x3, not 15:9 like the first.
Until such a time as @HyperspaceMadness adds an explicit aspect ratio for the second screen, this is as good as we can get.
Although not as good as the Mega Bezel (It uses my standard overlay) my koko-aio 3ds preset should work if you set the custom aspect ratio correctly.
Thx so much for taking the time and confirming that what I’m trying to do is simply not possible right now. I’ll check out your settings and also the koko-aio 3ds preset. This may be good enough for me to just leave it for the time being.
Are there plans to add more options for the second screen? Maybe in the HSM 2.0 update?
That would be a question for @HyperspaceMadness. There may well be a very good reason he didn’t add more options, or it could just be that they aren’t needed in Retroarch.
If adding them is possible, I’m not sure that letting Shaderglass define his roadmap is appropriate.
What I can tell you is that I’m sure he never considered the dual screen method " complete".
Use the “Flip Viewport Vertical” parameter.
Would it be possible to get a wood version of TV1? Basically I want to use a wood version for console games and the normal black/gray version for arcade. Technically I could do already this with TV7 since it has both plastic and wood, but then on Arcade games I’ll have the channel select panel in the top right
Sure! 
I do have the new Super Famicom waiting to commit, so the timing is just right.
BTW Everyone… to accommodate ShaderGlass, I’ve decided to create a new repo to host presets for systems that Retroarch doesn’t support.
I am going to rework my Batocera XBOX overlay, add a couple of alternates, and let it sit by itself in the repo until I do some research on more systems.
Thanks, Duimon… I would probably use that basic wood TV background as well for a few things.
@Duimon you are THE best! Thanks for doing this =)
I already tried ShaderGlass with Xemu & Dolphin, and it works great. Dolphin requires a different setup depending if you want to use the 16:9 or the 4:3 aspect ratio shaders, but both work.
As you know Azahar 3DS did not work 100%, but maybe there is a way in the future.
@Duimon Any chance you could make a version of your overlays from the github that would work with just the simple crt-geom shader? I was trying to use a pvm overlay, and i have no idea how to get the screen to shrink and move up to be into the pvm monitor on those overlays. Maybe in addition to having those overlays on their own for those of us who can’t run the HSM reflection shaders well yet, combined with shader presets for some standard shaders would be a good base-setup entry point?
With no offense intended, I think the issue is with your knowledge… not the overlays themselves.
You can read through this guide.
Creating_An_Overlay_Using_The_Mega_Bezel.md
Specifically, the “3. Getting Retroarch Custom Aspect Coordinates” section, which details multiple methods of acquiring overlay coordinates.
Keep in mind that my overlays are 4K (3840x2160) and will have to be, at least temporarily, resized if your display is not 4K.
The 4k overlays can be used on a 1080 display, but will need 1080 coordinates.
Absolutely no offense taken. So, i looks like advanced positioning settings are in the shader menu itself? I think that’s what you are showing in that guide. I was looking in every other place but there. Oh wait,…I could just load your actual shaders for the pvm monitor bezels, and then copy those position settings down and just apply them to another shader right?
Nope. Standalone shaders don’t have scaling features.
There is the resolution info in the Mega Bezel and it shows region info.
But it doesn’t match what I get using my GIMP method.
Most likely because it doesn’t count the “Black Line” on the screen edge as part of the region, which is fine if you want a black border, but it is much easier to match curvature in a standalone shader if you can hide behind the overlay. (You don’t have to get it perfect.)
You need to read the entire guide. The beginning is all about creating an overlay from a Mega Bezel shader preset. (Which I already did to create my overlays.)
The advanced positioning is under “Video>>Scaling>>Aspect Ratio>>Custom” in the Retroarch settings menu.
I manually edit my core or game config as detailed in the aforementioned guide. Doing it live inside the Retroarch interface is cumbersome and unnecessary.
Setting up overlays is tiresome. This difficulty is one of the main inspirations behind the Mega Bezel. (Beyond the perfectly executed reflection.)
Cool, thanks! I managed to get a little better performance from the HSM shader pack of yours now, so I can roll with that. My Sega Saturn has small black lines on the left and right though, even just loading your shader and doing nothing else to it. I moved the custom aspect ratio around, but that just moved the entire picture, including bezel, around with it. So I reverted. I can live with that if need be. Thanks for your replies!
You can use the cropping parameters to get rid of the black lines.
Thank you so much. All this work is absolutely phenomenal!
