You're assuming that the camera would be mounted to the tripod, and that there ought to be a way to mount the lens as well. That's not the case. When you're using a large, heavy lens with its own tripod mount/collar, you only mount the lens to the tripod. The camera simply hangs off the lens. Don't worry -- the lens mount will be able to support the camera.

There are cases with extremely long lenses (long as in focal length as well as long physically), you might want to use two tripods to make the setup as stable as possible. When the focal length is a meter (1000mm) or more, it doesn't take a whole lot of camera motion to create a great deal of blur in the image. As you've intuited, that's a fussy, awkward undertaking, so it's not something you'd do for the heck of it, or if you need to be able to move the point of aim; it's much more common to use the photographer as the secondary anchor point if the shutter speed can remain reasonably high (that is, you'd mount the lens to your tripod/stand but still hold the camera).