Is there a way to split the reference view? (Not before/after)

I could be wrong, but I don’t think there is a way to split the Reference or Compare views that way. Instead of comparing spatially, would it work instead to compare temporally (over time)? For example, as shown in the demo below, would it work for you to switch among frames while locking the view to a single region? If one frame is inconsistent at the region of interest, it should be easy to spot.

If this is something that will work for you, a couple of tips should make it work a little better:

• Choose View > Lock Zoom Position to prevent the zoom location from changing from frame to frame. 
• If you want to do this in the Library module, it will work best if you pre-build 1:1 previews for the images so that each frame displays without delay. In the Develop module, how quickly it redraws will depend on how new and powerful your system is. 

If you need to do this often with hundreds of images, you might look into whether the command Settings > Match Total Exposures might give you a head start on leveling the exposures and save some time. If that doesn’t help much, you can look into something like LrTimelapse (paid software) that provides a lot of precise control over time lapse leveling and ramping that just doesn’t exist in Lightroom Classic itself, so that you don’t have do frame by frame tweaks. And it saves its edits to the same XMP metadata files that Lightroom Classic uses, so that edits can be passed between the two applications for further editing in either.