You can change the models by dragging a different hand prefab to the "Left Hand Graphics Model" and "Right Hand Graphics Model" properties (in the inspector) for the HandController object in your scene.
The HandControllerSandbox already contains a HandController. By adding a second, you have effectively added another set of hands to the scene. You can just delete the second HandController object.
As for hands passing through objects, the hands aren't designed to be blocked by other objects. If you want the hands to be stopped by a wall, for example, you would have to add that feature. (I don't know how hard or easy that would be to do.) You can also set up the scene so that the walls are at the edge of the hand's area of motion so that the intersections are minimized.