The last time I checked ( years ago ) a texture applied to several entities would be a single surface, provided they used compatible texture flags.
I remember testing this with the castle demo. I added some code to load one of the textures used in the castle. Then I wrote some new data to the texture buffer of my loaded texture. This also changed the appearance of the castle, so it was using the same texture, not a separate copy.
I'm not sure that result tells us very much about surface usage... you could create two separate cubes and give them the same texture and this texture would alter on both objects when you wrote to it, but both objects would still have their own surface.
Before it got bogged down in miscommunication, I posted some exampler code in this thread that might prove illustrative of how things seem to work.