Engine layouts

How about two of those on a single flat-plane crank?

Apparently DUHC engines are actually a thing that exist(ed).

Pic related is a Talbot Lago engine.
I wonder why it's not more common. With a gear drive it would be robust and allow the exhaust and intake cam to be adjusted independently.

isn't a Bugatti 8.0 basically 4 of these?

Yeah, it's like 2 of them sandwiched front-to-back with 4 turbos strapped to it.