They're too different, and too long, to be released a double feature
You do realize that each movie was only like 70 minutes in the American version? With the actual trailers, fake trailers, and both films, it was still only about 3 hours. That's, for example, about as long as The Two Towers, or Dances with Wolves. Or a half an hour shorter than the theatrical Return of the King... Or about 10 minutes longer than the most recent Pirates of the Caribbean. As an aside, typical double features in the US in the old days were 80-90 minute films.
Too different, maybe, but I really can't see how you can argue they're too long. It just makes me sad double features are so rare these days (some small non-chain theatres in the US still do them once in a while). It's nice to actually be able to call an "evening at the movies" just that once in a while.