combing areas from two DNG’s pre-Lightroom processing

As others have explained, you can't splice RAW files in LR, but a correctly set up RAW file will look identical in .tif or .psd format once opened in PS. I'd suggest this workflow:

1. Set LR external editing prefs to Adobe 98 col space / 64bit / save to .tif or .psd / high res (most people choose 300ppi, but 360ppi better in many printed scenarios).
2. Process both DNG RAWs in LR > use Reference View to ensure colour consistency > DeNoise on both > R-click > Open as Layers in PS > ensure open eyes layer on top > auto (or manually) align layers so eyes match > add inverted Mask to open eyes layer > brush in open eyes > sharpen if required (use seperate merged layer) > save > automatically reappears in LR (useful for project management / exporting multiple files in uniform format).
3. You might want to consider a coding system to easily differentiate between PS edited files saved back to LR, and LR RAWs. (I do this by tagging blue in LR and auto-renaming external edits to denote app used, eg: name_original_file_number-PS.tif for files externally edited in Photoshop, or name_original_file_number-TPZ.tif for files edited in Topaz.

"Splice together sections of two DNG files" is not a feature of Lightroom Classic. This has to be done in Photoshop.