2fort's boundary doesn't start for quite a while past the boundaries of the map, it's always best to have this sort of "buffer zone" as it gives you some room to make the map > 3D skybox transition a bit more believable.
Everything beyond the map though (the farm fields and clouds in the distance) are handled by the 3D skybox in 2fort.
No you cannot have more than one 3D skybox.
EDIT: Here's a screenshot of 2fort's "buffer zone", the pink arrows highlight the seam between the actual map and the 3D skybox.
As you can see the geometry and detail in the 3D skybox is rather simple compared to the actual map. It's a good idea to have little visual noise in the skybox to help keep players' attention on the action.