Why can't I paint a tile on top of another tile in the same tilemap without overriding the first one?

So this is what’s happening, take a look:

See? If I’m trying to paint a tile over another tile inside the same tilemap, I can’t. I can only put one tile at a time, I can’t paint on top of it unless I make a new tilemap. If I can’t do this, then what’s the point of the “Sort Order” option of the Tilemap Renderer?

I hope you can shed some light on this. Thanks for reading.

Okay, so the tilemap sorting is working as long as you are not using the same tile, but it’d be nice if you could put multiple sprites in one tile without creating a new tilemap.