Would it be possible to sandwich 4 car HEx sets between 2x GWR 387s, to allow the Paddington - Didcot, (and hopefully soon Paddington - Swindon) sets to go from 8 cars to 12, so keeping conventional cabs on the ends, and allowing them to be driven by GWR drivers, or are the HEx sets no longer compatible?
The HEx sets are no longer compatible. Even if they were, you’re screwing yourself over if something crops up and you need to uncouple for any reason