You are explaining the right thing in the most horribly wrong way. There is no "both sectors" when you join them, there is only one sector at this point and it's all due to the line references. Remember sectors don't exist tangibly in Doom maps, so what joining does is make all the lines share one sector reference across all selected lines, regardless of where they are.Nevander wrote:When you join sectors, they both become the same sector as far as the map is concerned, which means they will both be at the same height and have the same flats and light level, but the sidedefs textures can be different. This is mostly used to "pack" sectors or create dummy sectors for use outside of the map.
Lines. You made the lines have the same sector numbers.Nevander wrote:So joining makes sectors share the same sector number