I had had the same thought...
Strangely enough though, even using Stems, AI, Phase cancellation etc. trying to separate the Lead and backing vocals just ends up with a muddy mess.
The mix has been so overlaid that even getting a clean sounding backing track without vocal artifact has proven pretty much impossible, let alone separating the backing and lead vocals.
John and Paul's vocals always did blend well, and in this case, UVR5, DeMIXPro and even Audionamix can't seem to break the mix apart to leave clean stems to work with. I even went back to the 2015 bootleg to try and lift out stems that could be pulled into the cleanest audio strip I could get, but without any real success. I can only assume that the mix has been so multi-tracked as to leave no clean parts for even AI to identify...