Timo said:
The actual line is "The klingons control deck six and starboard deck seven... while we control everything above."
Which sort of makes one think that the decks in this episode are supposed to be numbered from bottom up. That is, the Klingons control six, and then half of seven above, but the heroes control the other half of seven. Why else would the Klingons fail to control the entire higher-numbered deck?
Clearly the "lower decks" idea has to be taken with a grain of salt because it would be very, very difficult to isolate 400 people from 38 by drawing any arbitrary horizontal line across the ship. A more complex arrangement of locked isolation bulkheads and doors has to be postulated - as in the diagram.
And remember that in "The Enemy Within" dialogue, "lower levels" are equated with "the engineering deck"...
So, while the location of Main Engineering can certainly be argued over, I'd tend to think that the makers of "Day of the Dove" actually believed in the secondary hull theory. Or at least those creating the graphics and refining the script did. The original writer might have been confused about deck numbering conventions and the like, though.
Timo Saloniemi