TVH is actually a combination of two scripts put together. Bennett wrote the film's beginning and end. Meyer has everything in the middle, including the bulk of the San Francisco scenes, as he used a lot of ideas that were in Time After Time.
The latter part of this is accurate. The former is not. Bennett and Meyer worked very closely in putting together the script for The Voyage Home
-- there was a draft put together by Steve Meerson and Peter Krikes, and it was tossed out (which is when Meyer got involved -- and then they went to WGA arbitration, similar to what happened with the credits for The Undiscovered Country).
The script for The Voyage Home
was not a mash-up, but rather a result of Meyer and Bennett collaborating. There is definitely a tonal difference between the acts, but it's not like they were acting independently of one another.
Don't believe all the bullshit in the Shatner books.