The primary problem with having a skybox wall isn't going to trying to figure out how to put a normal texture alongside it; there are ways of overcoming that. The primary problem is going to be having the map behind a skybox brush. In order to understand the problem, you need to understand how
visibility works in Source. In Source, the map is divided up into sections called "visleafs", and during the VVIS compile process, the compiler figures out which leaves can see each other (that is, which leaves can be connected via a straight line that does not cross world geometry). This way, Source doesn't need to do a line-of-sight test to every object in the map during runtime (which would take more processing power than it saves). However, sometimes objects behind a wall will still be rendered. In ordinary cases, the wall is rendered on top of those objects. But the skybox is a special case. The 3D and 2D skyboxes are always rendered behind
everything else, no matter what, as long as a brush textured with a toolsskybox* texture is visible. In an improperly constructed map, this can cause objects behind a skybox wall to be visible on top of the skybox when it's undesired. So be careful when including objects behind a skybox wall, as players may accidentally see what's behind it.