Is there a way using the Compositor to get a Split Viewer that allows comparing the Filmic version against the AgX version?
Yep! Assuming you’re using one of the recent AgX versions, or the upcoming Blender release that has it natively, both transforms are still implemented, so you can just choose both.
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The setting of the Blender color management seems to be important.
It’s not easy to match the original 
AGX version used
You don’t want to use a view transform in the Color Management settings if you’re doing it manually in the compositor, that’ll result in a double transform: sRGB out of the compositor, then another sRGB transform on its way to the display.
You’d want to
A) set the transform in the Properties to None, accepting that you’re just going to do all transforming yourself, or
B) add another transform at the end of the compositing chain, converting sRGB to linear, so when the final transform comes along and converts linear to sRGB the values end up right back where they were. (If your final transform is Standard, aka Linear > sRGB, then you want that transform to be sRGB > linear, if it’s Filmic, aka Linear > Filmic sRGB, you’d want that transform to be Filmic sRGB > Linear, and so on. Just the reverse of the transform that’s about to be applied.)
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Also, what’s going on with your colorspace conversions there? If that image is a view-transformed render you want to apply an alternate transform to, you need to “unwrap” the original transform (which is only sort of possible, since the view transform is destructive, so you’ve lost information you can’t get back) and then apply a new one: so if that image was rendered with Filmic, you’d apply Filmic sRGB as the input transform (if you rendered it with a different contrast look, tough luck), which will take the values back into linear space, then you could apply, for example, AgX with a Convert Colorspace set to Linear > AgX sRGB. Or Filmic, which would round-trip it right back to where it was. If it was rendered with AgX, same thing, except the transform you need to “unwrap” is AgX instead of Filmic.
You can also do it directly in one Convert node: Filmic sRGB to AgX sRGB, or whatever, which I think is what you’re trying to do there? But in that case, you can’t transform the color before that, which is what’s happening with that sRGB-to-linear transform on the Image node, before it even gets to the conversion node.
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The view transform in the Color Management properties is still active, so you’re getting a double transform: one side of the split is getting Filmic, one side’s getting AgX, and then both sides are getting another AgX, at the project level.
Like I said in my previous response up there, you’ll want to either set the overall transform to Raw, since you’re handling the transforming yourself, or add another AgX → linear transform at the end of the comp, to prepare it for the linear → AgX transform it’s about to get. Personally, I’d recommend the first, for less messing with the color data: why put the values through more stress than they need and risk breaking something
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There’s no none menu… 
The standard is closest to the original.
Ah, yeah, “Raw,” then, sorry. I remember there bring some debate in the dev threads about which name to use, personally I think “None” is clearer, but eh!
You want no transform happening at all, since you’re doing the transforms yourself already, in the compositor.
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What are you trying to do, specifically? Thinsoldier’s original post was about splitscreening between AgX and Filmic, which I explained in my previous responses—if you follow those same steps, you’ll get those same results. So what’s your goal?
I think I misunderstood because I am not familiar with English.
It’s not the problem I want to solve. sory…
Sure, so what is? Happy to help, especially if it’s related to the original question—I’m sure we won’t get in trouble for solving a second, related issue here in the thread for the first one.
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Specifically I want the live 3d viewport render to be in a split view comparison between AgX and Filmic (or old Standard vs Whatever the AgX alternative to standard will eventually be). I’m not using imported image files.
Raw worked for me.
Yes, sorry, I was addressing Kim! I understood your question, and it seems we found a solid solution for you, I was asking what their specific goal was since it seems to be something tangential to what you and I were originally talking about
uhhh, which “Linear.xxxxx” in Blender 4 matches “Linear” in Blender 3.6?
I can’t get this 3.6 test to recreate in 4.0
Should be Linear Rec.709. What’s happening in 4.0?
Yes it’s rec.709. Seems to be some kind of bug in the compositor. When I opened the file at one point both sides of the split viewer looked wrong. Another time they looked correct but when applied to the final render it looked wrong. Another time there were 4 different incorrect appearances, 2 in the viewport, 2 in the final render.