That would mean that those two hidden parts are actually forming one hidden map, right?
Yes, probably. Almost all maps in SD Snatcher have its parts/rooms stored in the same map.
Furthermore, the walls are pretty similars in both parts of "Survival World"
Here is the complete map:
Cool manuel: Btw did you also find a way a cheat to access all areas (with the tricycle)
Yes, changing a number in the saved data all areas are unlocked. But I don't remember the offset it right now 
EDIT: Perhaps it was #FC0 (1st save), #8FC0 (2nd), etc... Just write #FF in your user disk at those offsets.
I have not checked everything. What I want to now is decode the original maps to find unkown locations, or just to see how are they stored 
Some exmaples:
Junker - Rooms
Junker HQ
Caves
Syd Garden
Great effort Guillian! This is really interesting
I recall a Sunrise Magazine on which was described how to get to that 'secret' level. As soon as you had to enter the Magical House in Syd Garden, you should push the disk eject button so that the disk would almost flip out of the drive, and sometimes it would load the 'secret area' instead. How dangerous it may sound, it never worked here. On that same magazine was a save state, which I copied onto my save disk and indeed, Gillian (Guillian? 
) was stuck in that Survival Land. Never got to see the top of the map (as seen in your PNG) there, because Gillian used to complain about getting through the door... But applying that walk-through-walls-cheat makes sense in that particular case
Maybe there is an hidden stage in the Konami building where we can meet Hideo Kojima and other people who are responsible for the making of the Snatcher/SD-Snatcher game.
That should be great