Yes, it can be done that way, but that's a RAM and HDD devourer. ._.
EDIT: HAHA! I'VE GOT IT! Since you need to use a combined tileset to combine maps, you can simply use two tilesets. One tileset is where the outer region is the main region. All other regions are darkened in the tileset itself. The other tileset is where the interior is the main region so the outer region is darkened. I can't remember exactly how I made it work, but I think as soon as you switch maps, the tileset is also switched. You basically need to have two tilesets for each interior/exterior combination where always one part is darkened.