My knowledge with multiple maps is massively overrated. I only can remember some things others had found out more than 20 years ago.
1. Each multiple maps scenario needs a standard map included.
2. The types of the secondary maps and the number of secondary maps is set in the rules text file.
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a) The type of the map is set by the number in the first vertical column of the screenshot. Number 6 is the standard (terran) map.
b) The horizontal columns under the title @SECONDARY_MAPS list the number of secondary maps in that scenario (and the characeristics of those maps)
As the original game extended has another standard (terran) map as its secondary map, here another such column has to be added to this entry for every other added secondary map. So for playing the original extended game with 4 planets you have to add 3 such secondary maps. Of course, if you want, you can add some non-standard maps for some of those planets, too.
3. The names for the new secondary maps can be created in the Game text file of the original extended scenario.
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Under @TRANSPORTOPTIONS the default settings for this scenario are Earth, Centaurus, Map 3 and Map 4. I renamed Map 3 and 4 for the following screenshots to Vulcan and Andor.
The multiple maps settings, even in the original extended scenario, can be set to premade maps, too. These maps must all have the same size.
4. Terrain settings for the new secondary maps:
The new terrain settings for the secondary maps must be added in the rules text (have a look at the handling in the SciFi game) and the terrain graphics for the secondary maps must be added (otherwise you will only see a grey screen for those maps). For 4 planets you need 8 terrain graphic files (in conventional Civ 2 ToT).
In the following, I post 4 screenshots for the original extended scenario with 4 imported big Earth maps from the first post in this thread (each one for one of the 4 planets in the new settings of this scenario), but I don´t think that such a game would provide a lot of fun:
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This all can be done easily, but now the real work is starting. The
transport relations between the 4 maps must be set for units, buildings, transporters and events - and this are really tons of work.