This is a newbie question ;) . Imagine a map where player goes from outside environment (with a sky) into a room. The floor texture transitions from grass to indoor-tiles, without any "bump": both outdoor and indoor floors are at the same height. How is this achieved?

I tried creating a floor brush onto a outdoor floor brush such that both surfaces are level, but have different textures. The problem with this major flickering of textures: I see both outdoor and indoor texture depending on the player position within the map. How do I make this work?
Thanks!