After doing some research and learning a bit about aspect ratios, here is the definitive answer to how split screen is handled throughout the MCC. For example purposes I will be using the resolution 1920x1080 for screen sizing arithmetic. Please note that I will be comparing MCC CE and MCC H3, and will only be interested in how split screen is handled in widescreen formats because AFAIK that is the only format the MCC natively supports. I make the assumption that CE and H2C work similarly as does every other title to H3. Let’s jump into it.
When Combat Evolved Anniversary was released on the 360, widescreen split screen support was silently added to the game. By extension this means it’s also present in the MCC version of CEA. As the OP has pointed out, CE takes full advantage of widescreen displays without adding black bars to the side of the screen like every other title except H2C. What exactly is going on here? As it turns out the camera for each player crops the top and bottom of the original image leaving a picture of 1920x540 resolution which is a 32:9 aspect ratio. Stacking both images creates a 1920x1080 image which is low and behold 16:9 aspect ratio. What’s neat is that because it’s a sub sample of the original image it can be displayed without any noticeable loss of image quality and fits the full width of the display for both players. 540p height is exactly half of 1080p. The down side of this method is that it removes 25% of the top and bottom of the image resulting in a total loss of 50% of the original image. Most of the important information is usually presented in the center of the image so the loss isn’t super noticeable, but can feel claustrophobic. CE does some extra shenanigans with FOV and AA to mitigate this effect, but I’ll discuss that later.
Now onto how Halo 3 and the other titles handle “wide screen” split screen. Effectively the cameras for both players scale the image down by a factor of 2 on both the width and height. This creates an image resolution of 960x540 which is still 16:9 aspect ratio, but note that it does NOT match the width of the original display value. Stacking both of these images creates the odd 8:9 aspect ratio. Black bars are present in the image because there simply isn’t enough image left after the scale to fill the full wide screen. The primary downsides to this method is the image quality degrades after the down scale creating a noticeable more blurry picture and fails to take full advantage of the display. The only upside to this method is that there is no loss of image like the cropping found in CE.
What I find odd is the neither CE nor H2C supported widescreen split screen, until their respective anniversary editions added this functionality. Halo 4 released after CEA and still failed to include this functionality. Even with the release of the MCC, only H2A had this feature added which is arguably the most visually taxing game in the entire collection.
The current system for H3 and other titles is probably the worst way of going about this. They can either replicate the same system of CE and crop 50% of the image to still take advantage of the larger display or they can still scale the image down, but instead of scaling a 1920x1080 image, they would need to scale down from 3840x1080 image to mitigate scaling artifacts and create a 1920x540 image which would match the full width of the display. Fun fact this how CE and H2C handled split screen resolution on their original releases with albeit much smaller resolutions.
Personally I would prefer the CE method for the sake of consistency but each have their pros and cons.
But what about FOV? Why does CE increase the players FOV in split screen from 86 to 108? Well to minimize the amount of image lost. Before you could adjust the FOVs in all titles, you could see significantly more on your left and right sides when split screening in CE. Arguably more than what the single player widescreen offered you. The resolution and aspect ratio remain the same, but the zoom of what you can see is very impressive in split screen.
If I had to guess why H3 and onward never got the same split screen treatment as CE and H2A, it’s probably because saber interactive (the developers responsible for CEA and H2A) didn’t work on porting the other titles to the collection. That was ruffian’s job. Hopefully 343 and saber can make this happen next year, I would love to take full advantage of my 4K display.