I am currently trying to improve speed on a tilemapped game. The tiles are all in one big texture. If two or more diffrent floor tiles are neighbours the system creates a transistion on one tile: The base floor tile is rendered first then the transition is rendered on top of the tile. The transistion is a tile with transparent parts in it. On top of this the walls of the game are rendered in a separate layer. Most of the wall field is transparent.

Currenty the engine uses multi-pass rendering which works - but the performance is quite bad. I want to improve this and now I try to use multi-texturing.

Currently I succeded in rendering two tiles above each other but the result is not what I expected. I get problems because of the transparency.
I think GL_DECAL is nearly what I want but there are problems if the floor-texture has transparent parts:
If I render the wall above the floor the wall is only visible if the floor is opaque. If the floor is transparent the wall disappears too.

I think I have to use GL_COMBINE but with which paramters?